免疫学（英文版）：Chapter 02

contrast to a unipotent cell, which differentiates into a single cell type, a hematopoietic stem cell is multipotent, or pluripo- tent, able to differentiate in various ways and thereby generate erythrocytes, granulocytes, monocytes, mast cells, lympho- cytes, and megakaryocytes. These stem cells are few, normally fewer than one HSC per 5 H11003 10 4 cells in the bone marrow. The study of hematopoietic stem cells is difficult both be- cause of their scarcity and because they are hard to grow in vitro. As a result, little is known about how their proliferation and differentiation are regulated. By virtue of their capacity for self-renewal, hematopoietic stem cells are maintained at stable levels throughout adult life; however, when there is an increased demand for hematopoiesis, HSCs display an enor- mous proliferative capacity. This can be demonstrated in mice whose hematopoietic systems have been completely de- stroyed by a lethal dose of x-rays (950 rads; one rad repre- sents the absorption by an irradiated target of an amount of radiation corresponding to 100 ergs/gram of target). Such ir- radiated mice will die within 10 days unless they are infused with normal bone-marrow cells from a syngeneic (genetically identical) mouse. Although a normal mouse has 3 H11003 10 8 bone-marrow cells, infusion of only 10 4 –10 5 bone-marrow cells (i.e., 0.01%–0.1% of the normal amount) from a donor is sufficient to completely restore the hematopoietic system, chapter 2 a73 Hematopoiesis a73 Cells of the Immune System a73 Organs of the Immune System a73 Systemic Function of the Immune System a73 Lymphoid Cells and Organs—Evolutionary Comparisons Cells and Organs of the Immune System T ?? ?????? ?????? ???????? ?? ???? ????????? organs and tissues that are found throughout the body. These organs can be classified functionally into two main groups. The primary lymphoid organs provide appropriate microenvironments for the development and maturation of lymphocytes. The secondary lymphoid organs trap antigen from defined tissues or vascular spaces and are sites where mature lymphocytes can interact effectively with that antigen. Blood vessels and lymphatic systems connect these organs, uniting them into a functional whole. Carried within the blood and lymph and populating the lymphoid organs are various white blood cells, or leuko- cytes, that participate in the immune response. Of these cells, only the lymphocytes possess the attributes of diversity, specificity, memory, and self/nonself recognition, the hall- marks of an adaptive immune response. All the other cells play accessory roles in adaptive immunity, serving to activate lymphocytes, to increase the effectiveness of antigen clear- ance by phagocytosis, or to secrete various immune-effector molecules. Some leukocytes, especially T lymphocytes, se- crete various protein molecules called cytokines. These mol- ecules act as immunoregulatory hormones and play important roles in the regulation of immune responses. This chapter describes the formation of blood cells, the properties of the various immune-system cells, and the functions of the lymphoid organs. Hematopoiesis All blood cells arise from a type of cell called the hematopoi- etic stem cell (HSC). Stem cells are cells that can differentiate into other cell types; they are self-renewing—they maintain their population level by cell division. In humans, hematopoiesis, the formation and development of red and white blood cells, begins in the embryonic yolk sac during the first weeks of development. Here, yolk-sac stem cells differen- tiate into primitive erythroid cells that contain embryonic hemoglobin. In the third month of gestation, hematopoietic stem cells migrate from the yolk sac to the fetal liver and then to the spleen; these two organs have major roles in hematopoiesis from the third to the seventh months of gesta- tion. After that, the differentiation of HSCs in the bone mar- row becomes the major factor in hematopoiesis, and by birth there is little or no hematopoiesis in the liver and spleen. It is remarkable that every functionally specialized, ma- ture blood cell is derived from the same type of stem cell. In Macrophage Interacting with Bacteria 8536d_ch02_024-056 9/6/02 9:00 PM Page 24 mac85 Mac 85:365_smm:Goldsby et al. / Immunology 5e: Cells and Organs of the Immune System CHAPTER 2 25 which demonstrates the enormous proliferative and differ- entiative capacity of the stem cells. Early in hematopoiesis, a multipotent stem cell differenti- ates along one of two pathways, giving rise to either a com- mon lymphoid progenitor cell or a common myeloid progenitor cell (Figure 2-1). The types and amounts of growth factors in the microenvironment of a particular stem cell or progenitor cell control its differentiation. During the development of the lymphoid and myeloid lineages, stem cells differentiate into progenitor cells, which have lost the T H helper cell T C cytotoxic T cell Natural killer (NK) cell Myeloid progenitor Lymphoid progenitor Hematopoietic stem cell Self- renewing B cell Dendritic cell T-cell progenitor B-cell progenitor Eosinophil Monocyte Neutrophil Basophil Platelets Erythrocyte Erythroid progenitor Megakaryocyte Eosinophil progenitor Granulocyte- monocyte progenitor Basophil progenitor Macrophage Dendritic cell VISUALIZING CONCEPTS FIGURE 2-1 Hematopoiesis. Self-renewing hematopoietic stem cells give rise to lymphoid and myeloid progenitors. All lym- phoid cells descend from lymphoid progenitor cells and all cells of the myeloid lineage arise from myeloid progenitors. Note that some dendritic cells come from lymphoid progenitors, others from myeloid precursors. 8536d_ch02_024-056 8/5/02 4:02 PM Page 25 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: 26 PART I Introduction capacity for self-renewal and are committed to a particular cell lineage. Common lymphoid progenitor cells give rise to B, T, and NK (natural killer) cells and some dendritic cells. Myeloid stem cells generate progenitors of red blood cells (erythro- cytes), many of the various white blood cells (neutrophils, eosinophils, basophils, monocytes, mast cells, dendritic cells), and platelets. Progenitor commitment depends on the acquisi- tion of responsiveness to particular growth factors and cy- tokines. When the appropriate factors and cytokines are present, progenitor cells proliferate and differentiate into the corresponding cell type, either a mature erythrocyte, a partic- ular type of leukocyte, or a platelet-generating cell (the megakaryocyte). Red and white blood cells pass into bone- marrow channels, from which they enter the circulation. In bone marrow, hematopoietic cells grow and mature on a meshwork of stromal cells, which are nonhematopoietic cells that support the growth and differentiation of hema- topoietic cells. Stromal cells include fat cells, endothelial cells, fibroblasts, and macrophages. Stromal cells influence the dif- ferentiation of hematopoietic stem cells by providing a hematopoietic-inducing microenvironment (HIM) con- sisting of a cellular matrix and factors that promote growth and differentiation. Many of these hematopoietic growth factors are soluble agents that arrive at their target cells by diffusion, others are membrane-bound molecules on the surface of stromal cells that require cell-to-cell contact be- tween the responding cells and the stromal cells. During in- fection, hematopoiesis is stimulated by the production of hematopoietic growth factors by activated macrophages and T cells. Hematopoiesis Can Be Studied In Vitro Cell-culture systems that can support the growth and differ- entiation of lymphoid and myeloid stem cells have made it possible to identify many hematopoietic growth factors. In these in vitro systems, bone-marrow stromal cells are cul- tured to form a layer of cells that adhere to a petri dish; freshly isolated bone-marrow hematopoietic cells placed on this layer will grow, divide, and produce large visible colonies (Figure 2-2). If the cells have been cultured in semisolid agar, their progeny will be immobilized and can be analyzed for cell types. Colonies that contain stem cells can be replated to produce mixed colonies that contain different cell types, in- cluding progenitor cells of different cell lineages. In contrast, progenitor cells, while capable of division, cannot be replated and produce lineage-restricted colonies. Various growth factors are required for the survival, pro- liferation, differentiation, and maturation of hematopoietic cells in culture. These growth factors, the hematopoietic cytokines, are identified by their ability to stimulate the for- mation of hematopoietic cell colonies in bone-marrow cultures. Among the cytokines detected in this way was a family of acidic glycoproteins, the colony-stimulating fac- tors (CSFs), named for their ability to induce the formation of distinct hematopoietic cell lines. Another important hematopoietic cytokine detected by this method was the gly- coprotein erythropoietin (EPO). Produced by the kidney, this cytokine induces the terminal development of erythro- cytes and regulates the production of red blood cells. Fur- ther studies showed that the ability of a given cytokine to signal growth and differentiation is dependent upon the presence of a receptor for that cytokine on the surface of the target cell—commitment of a progenitor cell to a particular differentiation pathway is associated with the expression of membrane receptors that are specific for particular cy- tokines. Many cytokines and their receptors have since been shown to play essential roles in hematopoiesis. This topic is explored much more fully in the chapter on cytokines (Chapter 11). FIGURE 2-2 (a) Experimental scheme for culturing hematopoietic cells. Adherent bone-marrow stromal cells form a matrix on which the hematopoietic cells proliferate. Single cells can be transferred to semisolid agar for colony growth and the colonies analyzed for differentiated cell types. (b) Scanning electron micrograph of cells Add fresh bone- marrow cells Culture in semisolid agar Adherent layer of stromal cells Visible colonies of bone-marrow cells (a) (b) in long-term culture of human bone marrow. [Photograph from M. J. Cline and D. W. Golde, 1979, Nature 277:180; reprinted by permission; ? 1979 Macmillan Magazines Ltd., micrograph cour- tesy of S. Quan.] 8536d_ch02_024-056 8/5/02 4:02 PM Page 26 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: Cells and Organs of the Immune System CHAPTER 2 27 Hematopoiesis Is Regulated at the Genetic Level The development of pluripotent hematopoietic stem cells into different cell types requires the expression of different sets of lineage-determining and lineage-specific genes at ap- propriate times and in the correct order. The proteins speci- fied by these genes are critical components of regulatory networks that direct the differentiation of the stem cell and its descendants. Much of what we know about the depen- dence of hematopoiesis on a particular gene comes from studies of mice in which a gene has been inactivated or “knocked out” by targeted disruption, which blocks the pro- duction of the protein that it encodes (see Targeted Disrup- tion of Genes, in Chapter 23). If mice fail to produce red cells or particular white blood cells when a gene is knocked out, we conclude that the protein specified by the gene is neces- sary for development of those cells. Knockout technology is one of the most powerful tools available for determining the roles of particular genes in a broad range of processes and it has made important contributions to the identification of many genes that regulate hematopoiesis. Although much remains to be done, targeted disruption and other approaches have identified a number of transcrip- tion factors (Table 2-1) that play important roles in hematopoiesis. Some of these transcription factors affect many different hematopoietic lineages, and others affect only a single lineage, such as the developmental pathway that leads to lymphocytes. One transcription factor that affects multi- ple lineages is GATA-2, a member of a family of transcription factors that recognize the tetranucleotide sequence GATA, a nucleotide motif in target genes. A functional GATA-2 gene, which specifies this transcription factor, is essential for the development of the lymphoid, erythroid, and myeloid lin- eages. As might be expected, animals in which this gene is disrupted die during embryonic development. In contrast to GATA-2, another transcription factor, Ikaros, is required only for the development of cells of the lymphoid lineage. Al- though Ikaros knockout mice do not produce significant numbers of B, T, and NK cells, their production of erythro- cytes, granulocytes, and other cells of the myeloid lineage is unimpaired. Ikaros knockout mice survive embryonic devel- opment, but they are severely compromised immunologi- cally and die of infections at an early age. Hematopoietic Homeostasis Involves Many Factors Hematopoiesis is a continuous process that generally main- tains a steady state in which the production of mature blood cells equals their loss (principally from aging). The average erythrocyte has a life span of 120 days before it is phagocytosed and digested by macrophages in the spleen. The various white blood cells have life spans ranging from a few days, for neu- trophils, to as long as 20–30 years for some T lymphocytes. To maintain steady-state levels, the average human being must produce an estimated 3.7 H11003 10 11 white blood cells per day. Hematopoiesis is regulated by complex mechanisms that affect all of the individual cell types. These regulatory mech- anisms ensure steady-state levels of the various blood cells, yet they have enough built-in flexibility so that production of blood cells can rapidly increase tenfold to twentyfold in re- sponse to hemorrhage or infection. Steady-state regulation of hematopoiesis is accomplished in various ways, which in- clude: a73 Control of the levels and types of cytokines produced by bone-marrow stromal cells a73 The production of cytokines with hematopoietic activity by other cell types, such as activated T cells and macrophages a73 The regulation of the expression of receptors for hematopoietically active cytokines in stem cells and progenitor cells a73 The removal of some cells by the controlled induction of cell death A failure in one or a combination of these regulatory mecha- nisms can have serious consequences. For example, abnormal- ities in the expression of hematopoietic cytokines or their receptors could lead to unregulated cellular proliferation and may contribute to the development of some leukemias. Ulti- mately, the number of cells in any hematopoietic lineage is set by a balance between the number of cells removed by cell death and the number that arise from division and differentiation. Any one or a combination of regulatory factors can affect rates of cell reproduction and differentiation. These factors can also determine whether a hematopoietic cell is induced to die. Programmed Cell Death Is an Essential Homeostatic Mechanism Programmed cell death, an induced and ordered process in which the cell actively participates in bringing about its own demise, is a critical factor in the homeostatic regulation of TABLE 2-1 Some transcription factors essential for hematopoietic lineages Factor Dependent lineage GATA-1 Erythroid GATA-2 Erythroid, myeloid, lymphoid PU.1 Erythroid (maturational stages), myeloid (later stages), lymphoid BM11 Myeloid, lymphoid Ikaros Lymphoid Oct-2 B lymphoid (differentiation of B cells into plasma cells) 8536d_ch02_024-056 8/5/02 4:02 PM Page 27 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: 28 PART I Introduction many types of cell populations, including those of the hematopoietic system. Cells undergoing programmed cell death often exhibit distinctive morphologic changes, collectively referred to as apoptosis (Figures 2-3, 2-4). These changes include a pronounced decrease in cell volume, modification of the cy- toskeleton that results in membrane blebbing, a condensa- tion of the chromatin, and degradation of the DNA into smaller fragments. Following these morphologic changes, an apoptotic cell sheds tiny membrane-bounded apoptotic bod- ies containing intact organelles. Macrophages quickly phago- cytose apoptotic bodies and cells in the advanced stages of apoptosis. This ensures that their intracellular contents, in- cluding proteolytic and other lytic enzymes, cationic pro- teins, and oxidizing molecules are not released into the surrounding tissue. In this way, apoptosis does not induce a local inflammatory response. Apoptosis differs markedly from necrosis, the changes associated with cell death arising from injury. In necrosis the injured cell swells and bursts, re- leasing its contents and possibly triggering a damaging in- flammatory response. Each of the leukocytes produced by hematopoiesis has a characteristic life span and then dies by programmed cell death. In the adult human, for example, there are about 5 H11003 10 10 neutrophils in the circulation. These cells have a life span of only a few days before programmed cell death is initiated. This death, along with constant neutrophil production, maintains a stable number of these cells. If programmed cell death fails to occur, a leukemic state may develop. Programmed cell death also plays a role in main- taining proper numbers of hematopoietic progenitor cells. For example, when colony-stimulating factors are re- moved, progenitor cells undergo apoptosis. Beyond hematopoiesis, apoptosis is important in such immuno- logical processes as tolerance and the killing of target cells by cytotoxic T cells or natural killer cells. Details of the mechanisms underlying apoptosis are emerging; Chapter 13 describes them in detail. NECROSIS APOPTOSIS Chromatin clumping Swollen organelles Flocculent mitochondria Mild convolution Chromatin compaction and segregation Condensation of cytoplasm Nuclear fragmentation Blebbing Apoptotic bodies Phagocytosis Phagocytic cell Apoptotic body Disintegration Release of intracellular contents Inflammation FIGURE 2-3 Comparison of morphologic changes that occur in apoptosis and necrosis. Apoptosis, which results in the programmed cell death of hematopoietic cells, does not induce a local inflamma- tory response. In contrast, necrosis, the process that leads to death of injured cells, results in release of the cells’ contents, which may in- duce a local inflammatory response. Go to www.whfreeman.com/immunology Animation Cell Death 8536d_ch02_024-056 9/6/02 9:00 PM Page 28 mac85 Mac 85:365_smm:Goldsby et al. / Immunology 5e: Cells and Organs of the Immune System CHAPTER 2 29 The expression of several genes accompanies apoptosis in leukocytes and other cell types (Table 2-2). Some of the proteins specified by these genes induce apoptosis, others are critical during apoptosis, and still others inhibit apop- tosis. For example, apoptosis can be induced in thymocytes by radiation, but only if the protein p53 is present; many cell deaths are induced by signals from Fas, a molecule pre- sent on the surface of many cells; and proteases known as caspases take part in a cascade of reactions that lead to apoptosis. On the other hand, members of the bcl-2 (B-cell lymphoma 2) family of genes, bcl-2 and bcl-X L encode pro- tein products that inhibit apoptosis. Interestingly, the first member of this gene family, bcl-2, was found in studies that were concerned not with cell death but with the uncon- trolled proliferation of B cells in a type of cancer known as B-lymphoma. In this case, the bcl-2 gene was at the break- point of a chromosomal translocation in a human B-cell lymphoma. The translocation moved the bcl-2 gene into the immunoglobulin heavy-chain locus, resulting in tran- scriptional activation of the bcl-2 gene and overproduction of the encoded Bcl-2 protein by the lymphoma cells. The resulting high levels of Bcl-2 are thought to help transform lymphoid cells into cancerous lymphoma cells by inhibit- ing the signals that would normally induce apoptotic cell death. Bcl-2 levels have been found to play an important role in regulating the normal life span of various hematopoietic cell lineages, including lymphocytes. A normal adult has about 5 L of blood with about 2000 lymphocytes/mm 3 for a total of about 10 10 lymphocytes. During acute infection, the lym- phocyte count increases 4- to 15-fold, giving a total lympho- cyte count of 40–50 H11003 10 9 . Because the immune system cannot sustain such a massive increase in cell numbers for an extended period, the system needs a means to eliminate un- needed activated lymphocytes once the antigenic threat has passed. Activated lymphocytes have been found to express lower levels of Bcl-2 and therefore are more susceptible to the induction of apoptotic death than are naive lymphocytes or (a) (c) (b) (d) FIGURE 2-4 Apoptosis. Light micrographs of (a) normal thymo- cytes (developing T cells in the thymus) and (b) apoptotic thymo- cytes. Scanning electron micrographs of (c) normal and (d) apoptotic thymocytes. [From B. A. Osborne and S. Smith, 1997, Jour- nal of NIH Research 9:35; courtesy B. A. Osborne, University of Mass- achusetts at Amherst.] 8536d_ch02_024-056 8/5/02 4:02 PM Page 29 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: 30 PART I Introduction memory cells. However, if the lymphocytes continue to be activated by antigen, then the signals received during activa- tion block the apoptotic signal. As antigen levels subside, so does activation of the block and the lymphocytes begin to die by apoptosis (Figure 2-5). Hematopoietic Stem Cells Can Be Enriched I. L. Weissman and colleagues developed a novel way of en- riching the concentration of mouse hematopoietic stem cells, which normally constitute less than 0.05% of all bone- marrow cells in mice. Their approach relied on the use of an- tibodies specific for molecules known as differentiation antigens, which are expressed only by particular cell types. They exposed bone-marrow samples to antibodies that had been labeled with a fluorescent compound and were specific for the differentiation antigens expressed on the surface of mature red and white blood cells (Figure 2-6). The labeled cells were then removed by flow cytometry with a fluorescence- activated cell sorter (see Chapter 6).After each sorting, the remain- ing cells were assayed to determine the number needed for restoration of hematopoiesis in a lethally x-irradiated mouse. As the pluripotent stem cells were becoming relatively more numerous in the remaining population, fewer and fewer cells were needed to restore hematopoiesis in this system. Because stem cells do not express differentiation antigens TABLE 2-2 Genes that regulate apoptosis Gene Function Role in apoptosis bcl-2 Prevents apoptosis Inhibits bax Opposes bcl-2 Promotes bcl-X L (bcl-Long) Prevents apoptosis Inhibits bcl-X S (bcl-Short) Opposes bcl-X L Promotes caspase (several Protease Promotes different ones) fas Induces apoptosis Initiates FIGURE 2-5 Regulation of activated B-cell numbers by apoptosis. Activation of B cells induces increased expression of cytokine recep- tors and decreased expression of Bcl-2. Because Bcl-2 prevents apop- tosis, its reduced level in activated B cells is an important factor in T H cellB cell Antigen Cytokine receptor ↓ Bcl-2 ↑ Cytokine receptors Cessation of, or inappropriate, activating signals Continued activating signals (e.g., cytokines, T H cells, antigen) B memory cellPlasma cell Activated B cell Apoptotic cell Cytokines making activated B cells more susceptible to programmed cell death than either naive or memory B cells. A reduction in activating signals quickly leads to destruction of excess activated B cells by apoptosis. Similar processes occur in T cells. 8536d_ch02_024-056 8/5/02 4:02 PM Page 30 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: Cells and Organs of the Immune System CHAPTER 2 31 known to be on developing and mature hematopoietic cells, by removing those hematopoietic cells that express known differentiation antigens, these investigators were able to obtain a 50- to 200-fold enrichment of pluripotent stem cells. To further enrich the pluripotent stem cells, the re- maining cells were incubated with various antibodies raised against cells likely to be in the early stages of hematopoiesis. One of these antibodies recognized a differentiation antigen called stem-cell antigen 1 (Sca-1). Treatment with this anti- body aided capture of undifferentiated stem cells and yielded a preparation so enriched in pluripotent stem cells that an aliquot containing only 30–100 cells routinely restored hematopoiesis in a lethally x-irradiated mouse, whereas more than 10 4 nonenriched bone-marrow cells were needed for restoration. Using a variation of this approach, H. Nakauchi and his colleagues have devised procedures that al- low them to show that, in 1 out of 5 lethally irradiated mice, a single hematopoietic cell can give rise to both myeloid and lymphoid lineages (Table 2-3). It has been found that CD34, a marker found on about 1% of hematopoietic cells, while not actually unique to stem cells, is found on a small population of cells that contains stem cells. By exploiting the association of this marker with stem cell populations, it has become possible to routinely en- rich preparations of human stem cells. The administration of human-cell populations suitably enriched for CD34 H11001 cells Restore hematopoiesis, mouse lives E Eo L P L B E N Differentiated cells M N P S P React with Fl-antibodies against Sca-1 Lethally irradiated mouse (950 rads) Restore hematopoiesis, mouse lives 2 × 10 5 unenriched cells 1 × 10 3 partly enriched cells 30–100 fully enriched cells (a) E Eo L P L B E N M N P P S React with Fl-antibodies to differentiation antigens S P P Stem cell Progenitor cells P Restore hematopoiesis, mouse lives Survival rate, % 100 10 1 10 2 10 3 10 4 10 5 Number of cells injected into lethally irradiated mouse Fully enriched cells Partly enriched cells Unenriched cells (b) FIGURE 2-6 Enrichment of the pluripotent stem cells from bone marrow. (a) Differentiated hematopoietic cells (white) are removed by treatment with fluorescently labeled antibodies (Fl-antibodies) specific for membrane molecules expressed on differentiated lin- eages but absent from the undifferentiated stem cells (S) and prog- enitor cells (P). Treatment of the resulting partly enriched preparation with antibody specific for Sca-1, an early differentiation antigen, re- moved most of the progenitor cells. M = monocyte; B = basophil; N = neutrophil; Eo = eosinophil; L = lymphocyte; E = erythrocyte. (b) Enrichment of stem-cell preparations is measured by their ability to restore hematopoiesis in lethally irradiated mice. Only animals in which hematopoiesis occurs survive. Progressive enrichment of stem cells is indicated by the decrease in the number of injected cells needed to restore hematopoiesis. A total enrichment of about 1000- fold is possible by this procedure. 8536d_ch02_024-056 8/5/02 4:02 PM Page 31 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: 32 PART I Introduction (the “H11001” indicates that the factor is present on the cell mem- brane) can reconstitute a patient’s entire hematopoietic sys- tem (see Clinical Focus). A major tool in studies to identify and characterize the human hematopoietic stem cell is the use of SCID (severe combined immunodeficiency) mice as in vivo assay systems for the presence and function of HSCs. SCID mice do not have B and T lymphocytes and are unable to mount adaptive immune responses such as those that act in the normal rejec- tion of foreign cells, tissues, and organs. Consequently, these animals do not reject transplanted human cell populations containing HSCs or tissues such as thymus and bone mar- row. It is necessary to use immunodeficient mice as surrogate or alternative hosts in human stem-cell research because there is no human equivalent of the irradiated mouse. SCID mice implanted with fragments of human thymus and bone marrow support the differentiation of human hematopoietic stem cells into mature hematopoietic cells. Different subpop- ulations of CD34 H11001 human bone-marrow cells are injected into these SCID-human mice, and the development of vari- ous lineages of human cells in the bone-marrow fragment is subsequently assessed. In the absence of human growth fac- tors, only low numbers of granulocyte-macrophage progeni- tors develop. However, when appropriate cytokines such as erythropoietin and others are administered along with CD34 H11001 cells, progenitor and mature cells of the myeloid, lymphoid, and erythroid lineages develop. This system has enabled the study of subpopulations of CD34 H11001 cells and the effect of human growth factors on the differentiation of var- ious hematopoietic lineages. Cells of the Immune System Lymphocytes are the central cells of the immune system, re- sponsible for adaptive immunity and the immunologic at- tributes of diversity, specificity, memory, and self/nonself recognition. The other types of white blood cells play impor- tant roles, engulfing and destroying microorganisms, pre- senting antigens, and secreting cytokines. Lymphoid Cells Lymphocytes constitute 20%–40% of the body’s white blood cells and 99% of the cells in the lymph (Table 2-4). There are approximately 10 11 (range depending on body size and age: ~10 10 –10 12 ) lymphocytes in the human body. These lym- phocytes continually circulate in the blood and lymph and are capable of migrating into the tissue spaces and lymphoid organs, thereby integrating the immune system to a high degree. The lymphocytes can be broadly subdivided into three populations—B cells, T cells, and natural killer cells—on the basis of function and cell-membrane components. Natural killer cells (NK cells) are large, granular lymphocytes that do not express the set of surface markers typical of B or T cells. Resting B and T lymphocytes are small, motile, nonphago- cytic cells, which cannot be distinguished morphologically. B and T lymphocytes that have not interacted with antigen— referred to as naive, or unprimed—are resting cells in the G 0 phase of the cell cycle. Known as small lymphocytes, these cells are only about 6 H9262m in diameter; their cytoplasm forms a barely discernible rim around the nucleus. Small lympho- cytes have densely packed chromatin, few mitochondria, and a poorly developed endoplasmic reticulum and Golgi appa- ratus. The naive lymphocyte is generally thought to have a short life span. Interaction of small lymphocytes with anti- gen, in the presence of certain cytokines discussed later, in- duces these cells to enter the cell cycle by progressing from G 0 into G 1 and subsequently into S, G 2 , and M (Figure 2-7a). As they progress through the cell cycle, lymphocytes enlarge into 15 H9262m-diameter blast cells, called lymphoblasts; these cells have a higher cytoplasm:nucleus ratio and more or- ganellar complexity than small lymphocytes (Figure 2-7b). Lymphoblasts proliferate and eventually differentiate into effector cells or into memory cells. Effector cells function in various ways to eliminate antigen. These cells have short life TABLE 2-3 Reconstitution of hematopoeisis by HSCs Number of Number of mice enriched HSCs reconstituted (%) 19of 41 (21.9%) 25of 21 (23.8%) 59of 17 (52.9%) 10 10 of 11 (90.9%) 20 4 of 4 (100%) SOURCE: Adapted from M. Osawa, et al. 1996. Science 273:242. TABLE 2-4 Normal adult blood-cell counts Cell type Cells/mm 3 % Red blood cells 5.0 H11003 10 6 Platelets 2.5 H11003 10 5 Leukocytes 7.3 H11003 10 3 Neutrophil 50–70 Lymphocyte 20–40 Monocyte 1–6 Eosinophil 1–3 Basophil H110211 Go to www.whfreeman.com/immunology Animation Cells and Organs of the Immune System 8536d_ch02_024-056 9/6/02 9:00 PM Page 32 mac85 Mac 85:365_smm:Goldsby et al. / Immunology 5e: Cells and Organs of the Immune System CHAPTER 2 33 Lymphoblast S (DNA synthesis) Effector cell G 0 (i.e., plasma cell) Memory cell G 0 Small, naive B lymphocyte G 0 Antigen activation induces cell cycle entry Cycle repeats Cell division M G 1 (gene activation) (a) (b) Small lymphocyte (T or B) 6 μm diameter Blast cell (T or B) 15 μm diameter Plasma cell (B) 15 μm diameter G 2 FIGURE 2-7 Fate of antigen-activated small lymphocytes. (a) A small resting (naive or unprimed) lymphocyte resides in the G 0 phase of the cell cycle. At this stage, B and T lymphocytes cannot be distinguished morphologically. After antigen activation, a B or T cell enters the cell cycle and enlarges into a lymphoblast, which under- goes several rounds of cell division and, eventually, generates effector cells and memory cells. Shown here are cells of the B-cell lineage. (b) Electron micrographs of a small lymphocyte (left) showing con- densed chromatin indicative of a resting cell, an enlarged lym- phoblast (center) showing decondensed chromatin, and a plasma cell (right) showing abundant endoplasmic reticulum arranged in concentric circles and a prominent nucleus that has been pushed to a characteristically eccentric position. The three cells are shown at different magnifications. [Micrographs courtesy of Dr. J. R. Goodman, Dept. of Pediatrics, University of California at San Francisco.] 8536d_ch02_024-056 8/5/02 4:02 PM Page 33 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: lines in the laboratory. Strikingly, these ES cells can be induced to generate many dif- ferent types of cells. Mouse ES cells have been shown to give rise to muscle cells, nerve cells, liver cells, pancreatic cells, and, of course, hematopoietic cells. Recent advances have made it possible to grow lines of human pluripotent cells. This is a development of considerable im- portance to the understanding of human development, and it has great therapeutic potential. In vitro studies of the factors that determine or influence the development of human pluripotent stem cells along one de- velopmental path as opposed to another will provide considerable insight into the factors that affect the differentiation of cells into specialized types. There is also great in- terest in exploring the use of pluripotent stem cells to generate cells and tissues that could be used to replace diseased or dam- aged ones. Success in this endeavor would be a major advance because transplanta- tion medicine now depends totally upon do- nated organs and tissues. Unfortunately, the need far exceeds the number of dona- tions and is increasing. Success in deriving practical quantities of cells, tissues, and or- gans from pluripotent stem cells would pro- vide skin replacement for burn patients, heart muscle cells for those with chronic heart disease, pancreatic islet cells for pa- tients with diabetes, and neurons for use in Parkinson’s disease or Alzheimer’s disease. The transplantation of hematopoietic stem cells (HSCs) is an important ther- apy for patients whose hematopoietic systems must be replaced. It has three major applications: 1. Providing a functional immune system to individuals with a genetically determined immunodeficiency, such as severe Stem-cell transplanta- tion holds great promise for the regener- ation of diseased, damaged, or defective tissue. Hematopoietic stem cells are al- ready used to restore hematopoietic cells, and their use is described in the clinic below. However, rapid advances in stem-cell research have raised the possi- bility that other stem-cell types, too, may soon be routinely employed for replace- ment of other cells and tissues. Two properties of stem cells underlie their utility and promise. They have the capac- ity to give rise to more differentiated cells, and they are self-renewing, because each division of a stem cell creates at least one stem cell. If stem cells are clas- sified according to their descent and de- velopmental potential, four levels of stem cells can be recognized: totipotent, pluripotent, multipotent, and unipotent. Totipotent cells can give rise to an en- tire organism. A fertilized egg, the zygote, is a totipotent cell. In humans the initial di- visions of the zygote and its descendants produce cells that are also totipotent. In fact, identical twins, each with its own pla- centa, develop when totipotent cells sepa- rate and develop into genetically identical fetuses. Pluripotent stem cells arise from totipotent cells and can give rise to most but not all of the cell types necessary for fe- tal development. For example, human pluripotent stem cells can give rise to all of the cells of the body but cannot generate a placenta. Further differentiation of pluripo- tent stem cells leads to the formation of multipotent and unipotent stem cells. Multipotent stem cells can give rise to only a limited number of cell types, and unipo- tent cells to a single cell type. Pluripotent cells, called embryonic stem cells, or sim- ply ES cells, can be isolated from early em- bryos, and for many years it has been possible to grow mouse ES cells as cell CLINICAL FOCUS Stem Cells—Clinical Uses and Potential Bone marrow Nerve cells Heart muscle cells Human pluripotent stem cells Pancreatic islet cells Human pluripotent stem cells can differentiate into a variety of different cell types, some of which are shown here. [Adapted from Stem Cells: A Primer, NIH web site http://www.nih.gov/news/stemcell/primer.htm. Micrographs (left to right): Biophoto Associates/Science Source/Photo Researchers; Biophoto Associates/Photo Researchers; AFIP/Science Source/Photo Researchers; Astrid & Hanns-Frieder Michler/Science Photo Library/Photo Researchers.] 34 PART I Introduction 8536d_ch02_024-056 8/5/02 4:02 PM Page 34 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: Cells and Organs of the Immune System CHAPTER 2 35 sible for individuals to store their own hematopoietic cells for transplantation to themselves at a later time. Currently, this procedure is used to allow cancer patients to donate cells before undergoing chemo- therapy and radiation treatments and then to reconstitute their hematopoietic system from their own stem cells. Hematopoietic stem cells are found in cell populations that display distinctive surface antigens. One of these antigens is CD34, which is present on only a small percentage (~1%) of the cells in adult bone marrow. An antibody specific for CD34 is used to select cells displaying this antigen, producing a population en- riched in CD34 H11001 stem cells. Various ver- sions of this selection procedure have been used to enrich populations of stem cells from a variety of sources. Transplantation of stem cell popula- tions may be autologous (the recipient is also the donor), syngeneic (the donor is genetically identical, i.e., an identical twin of the recipient), or allogeneic (the donor and recipient are not genetically identical). In any transplantation procedure, genetic differences between donor and recipient can lead to immune-based rejection reac- tions. Aside from host rejection of trans- planted tissue (host versus graft), lymphocytes in the graft can attack the re- cipient’s tissues, thereby causing graft- versus-host disease (GVHD), a life- threatening affliction. In order to suppress rejection reactions, powerful immunosup- pressive drugs must be used. Unfortu- nately, these drugs have serious side effects, and immunosuppression in- creases the patient’s risk of infection and further growth of tumors. Consequently, HSC transplantation has fewest complica- tions when there is genetic identity be- tween donor and recipient. At one time, bone-marrow transplanta- tion was the only way to restore the hematopoietic system. However, the essen- tial element of bone-marrow transplanta- tion is really stem-cell transplantation. Fortunately, significant numbers of stem cells can be obtained from other tissues, such as peripheral blood and umbilical-cord blood (“cord blood”). These alternative sources of HSCs are attractive because the donor does not have to undergo anesthesia and the subsequent highly invasive proce- dure that extracts bone marrow. Many in the transplantation community believe that pe- ripheral blood will replace marrow as the major source of hematopoietic stem cells for many applications. To obtain HSC-en- riched preparations from peripheral blood, agents are used to induce increased num- bers of circulating HSCs, and then the HSC- containing fraction is separated from the plasma and red blood cells in a process called leukopheresis. If necessary, further purification can be done to remove T cells and to enrich the CD34 H11001 population. Umbilical cord blood already contains a significant number of hematopoietic stem cells. Furthermore, it is obtained from pla- cental tissue (the “afterbirth”) which is nor- mally discarded. Consequently, umbilical cord blood has become an attractive source of cells for HSC transplantation. Al- though HSCs from cord blood fail to en- graft somewhat more often than do cells from peripheral blood, grafts of cord blood cells produce GVHD less frequently than do marrow grafts, probably because cord blood has fewer mature T cells. Beyond its current applications in can- cer treatment, many researchers feel that autologous stem-cell transplantation will be useful for gene therapy, the introduction of a normal gene to correct a disorder caused by a defective gene. Rapid ad- vances in genetic engineering may soon make gene therapy a realistic treatment for genetic disorders of blood cells, and hematopoietic stem cells are attractive ve- hicles for such an approach. The therapy would entail removing a sample of hematopoietic stem cells from a patient, inserting a functional gene to compensate for the defective one, and then reinjecting the engineered stem cells into the donor. The advantage of using stem cells in gene therapy is that they are self renewing. Con- sequently, at least in theory, patients would have to receive only a single injection of en- gineered stem cells. In contrast, gene ther- apy with engineered mature lymphocytes or other blood cells would require periodic injections because these cells are not ca- pable of self renewal. combined immunodeficiency (SCID). 2. Replacing a defective hematopoietic system with a functional one to cure some patients who have a life- threatening nonmalignant genetic disorder in hematopoiesis, such as sickle-cell anemia or thalassemia. 3. Restoring the hematopoietic system of cancer patients after treatment with doses of chemotherapeutic agents and radiation so high that they destroy the system. These high-dose regimens can be much more effective at killing tumor cells than are therapies that use more conventional doses of cytotoxic agents. Stem-cell transplantation makes it possible to recover from such drastic treatment. Also, certain cancers, such as some cases of acute myeloid leukemia, can be cured only by destroying the source of the leukemia cells, the patient’s own hematopoietic system. Restoration of the hematopoietic sys- tem by transplanting stem cells is facili- tated by several important technical considerations. First, HSCs have extraordi- nary powers of regeneration. Experiments in mice indicate that only a few—perhaps, on occasion, a single HSC—can com- pletely restore the erythroid population and the immune system. In humans it is neces- sary to administer as little as 10% of a donor’s total volume of bone marrow to provide enough HSCs to completely re- store the hematopoietic system. Once in- jected into a vein, HSCs enter the circulation and find their own way to the bone marrow, where they begin the process of engraftment. There is no need for a sur- geon to directly inject the cells into bones. In addition, HSCs can be preserved by freezing. This means that hematopoietic cells can be “banked.” After collection, the cells are treated with a cryopreservative, frozen, and then stored for later use. When needed, the frozen preparation is thawed and infused into the patient, where it re- constitutes the hematopoietic system. This cell-freezing technology even makes it pos- 8536d_ch02_024-056 8/7/02 8:25 AM Page 35 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: 36 PART I Introduction spans, generally ranging from a few days to a few weeks. Plasma cells—the antibody-secreting effector cells of the B- cell lineage—have a characteristic cytoplasm that contains abundant endoplasmic reticulum (to support their high rate of protein synthesis) arranged in concentric layers and also many Golgi vesicles (see Figure 2-7). The effector cells of the T-cell lineage include the cytokine-secreting T helper cell (T H cell) and the T cytotoxic lymphocyte (T C cell). Some of the progeny of B and T lymphoblasts differentiate into mem- ory cells. The persistence of this population of cells is respon- sible for life-long immunity to many pathogens. Memory cells look like small lymphocytes but can be distinguished from naive cells by the presence or absence of certain cell- membrane molecules. Different lineages or maturational stages of lymphocytes can be distinguished by their expression of membrane mole- cules recognized by particular monoclonal antibodies (anti- bodies that are specific for a single epitope of an antigen; see Chapter 4 for a description of monoclonal antibodies). All of the monoclonal antibodies that react with a particular mem- brane molecule are grouped together as a cluster of dif- ferentiation (CD). Each new monoclonal antibody that recognizes a leukocyte membrane molecule is analyzed for whether it falls within a recognized CD designation; if it does not, it is given a new CD designation reflecting a new mem- brane molecule. Although the CD nomenclature was origi- nally developed for the membrane molecules of human leukocytes, the homologous membrane molecules of other species, such as mice, are commonly referred to by the same CD designations. Table 2-5 lists some common CD mole- cules (often referred to as CD markers) found on human lymphocytes. However, this is only a partial listing of the more than 200 CD markers that have been described. A com- plete list and description of known CD markers is in the ap- pendix at the end of this book. The general characteristics and functions of B and T lym- phocytes were described in Chapter 1 and are reviewed briefly in the next sections. These central cells of the immune system will be examined in more detail in later chapters. B LYMPHOCYTES The B lymphocyte derived its letter designation from its site of maturation, in the bursa of Fabricius in birds; the name turned out to be apt, for bone marrow is its major site of mat- uration in a number of mammalian species, including hu- mans and mice. Mature B cells are definitively distinguished from other lymphocytes by their synthesis and display of membrane-bound immunoglobulin (antibody) molecules, TABLE 2-5 Common CD markers used to distinguish functional lymphocyte subpopulations T CELL CD designation * Function B cell T H T C NK cell CD2 Adhesion molecule; signal transduction H11002H11001 H11001 H11001 CD3 Signal-transduction element of T-cell H11002H11001 H11001 H11002 receptor CD4 Adhesion molecule that binds to class II H11002H11001 H11002 H11002 MHC molecules; signal transduction (usually) (usually) CD5 Unknown H11001H11001 H11001 H11002 (subset) CD8 Adhesion molecule that binds to class I H11002H11002 H11001 H11001 MHC molecules; signal transduction (usually) (usually) (variable) CD16 (FcH9253RIII) Low-affinity receptor for Fc region of IgG H11002H11002 H11002 H11001 CD21 (CR2) Receptor for complement (C3d) and H11001H11002 H11002 H11002 Epstein-Barr virus CD28 Receptor for co-stimulatory B7 molecule H11002H11001 H11001 H11002 on antigen-presenting cells CD32 (FcH9253RII) Receptor for Fc region of IgG H11001H11002 H11002 H11002 CD35 (CR1) Receptor for complement (C3b) H11001H11002 H11002 H11002 CD40 Signal transduction H11001H11002 H11002 H11002 CD45 Signal transduction H11001H11001 H11001 H11001 CD56 Adhesion molecule H11002H11002 H11002 H11001 * Synonyms are shown in parentheses. 8536d_ch02_024-056 8/5/02 4:02 PM Page 36 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: Cells and Organs of the Immune System CHAPTER 2 37 which serve as receptors for antigen. Each of the approxi- mately 1.5 H11003 10 5 molecules of antibody on the membrane of a single B cell has an identical binding site for antigen. Among the other molecules expressed on the membrane of mature B cells are the following: a73 B220 (a form of CD45) is frequently used as a marker for B cells and their precursors. However, unlike antibody, it is not expressed uniquely by B-lineage cells. a73 Class II MHC molecules permit the B cell to function as an antigen-presenting cell (APC). a73 CR1 (CD35) and CR2 (CD21) are receptors for certain complement products. a73 FcH9253RII (CD32) is a receptor for IgG, a type of antibody. a73 B7-1 (CD80) and B7-2 (CD86) are molecules that interact with CD28 and CTLA-4, important regulatory molecules on the surface of different types of T cells, including T H cells. a73 CD40 is a molecule that interacts with CD40 ligand on the surface of helper T cells. In most cases this interaction is critical for the survival of antigen- stimulated B cells and for their development into antibody-secreting plasma cells or memory B cells. Interaction between antigen and the membrane-bound anti- body on a mature naive B cell, as well as interactions with T cells and macrophages, selectively induces the activation and differentiation of B-cell clones of corresponding specificity. In this process, the B cell divides repeatedly and differentiates over a 4- to 5-day period, generating a population of plasma cells and memory cells. Plasma cells, which have lower levels of membrane-bound antibody than B cells, synthesize and secrete antibody. All clonal progeny from a given B cell se- crete antibody molecules with the same antigen-binding specificity. Plasma cells are terminally differentiated cells, and many die in 1 or 2 weeks. T LYMPHOCYTES T lymphocytes derive their name from their site of matura- tion in the thymus. Like B lymphocytes, these cells have membrane receptors for antigen. Although the antigen- binding T-cell receptor is structurally distinct from im- munoglobulin, it does share some common structural features with the immunoglobulin molecule, most notably in the structure of its antigen-binding site. Unlike the mem- brane-bound antibody on B cells, though, the T-cell receptor (TCR) does not recognize free antigen. Instead the TCR rec- ognizes only antigen that is bound to particular classes of self-molecules. Most T cells recognize antigen only when it is bound to a self-molecule encoded by genes within the major histocompatibility complex (MHC). Thus, as explained in Chapter 1, a fundamental difference between the humoral and cell-mediated branches of the immune system is that the B cell is capable of binding soluble antigen, whereas the T cell is restricted to binding antigen displayed on self-cells. To be recognized by most T cells, this antigen must be displayed to- gether with MHC molecules on the surface of antigen-pre- senting cells or on virus-infected cells, cancer cells, and grafts. The T-cell system has developed to eliminate these al- tered self-cells, which pose a threat to the normal functioning of the body. Like B cells, T cells express distinctive membrane mole- cules. All T-cell subpopulations express the T-cell receptor, a complex of polypeptides that includes CD3; and most can be distinguished by the presence of one or the other of two membrane molecules, CD4 and CD8. In addition, most ma- ture T cells express the following membrane molecules: a73 CD28, a receptor for the co-stimulatory B7 family of molecules present on B cells and other antigen- presenting cells a73 CD45, a signal-transduction molecule T cells that express the membrane glycoprotein molecule CD4 are restricted to recognizing antigen bound to class II MHC molecules, whereas T cells expressing CD8, a dimeric membrane glycoprotein, are restricted to recognition of anti- gen bound to class I MHC molecules. Thus the expression of CD4 versus CD8 corresponds to the MHC restriction of the T cell. In general, expression of CD4 and of CD8 also defines two major functional subpopulations of T lymphocytes. CD4 H11001 T cells generally function as T helper (T H ) cells and are class-II restricted; CD8 H11001 T cells generally function as T cyto- toxic (T C ) cells and are class-I restricted. Thus the ratio of T H to T C cells in a sample can be approximated by assaying the number of CD4 H11001 and CD8 H11001 T cells. This ratio is approxi- mately 2:1 in normal human peripheral blood, but it may be significantly altered by immunodeficiency diseases, autoim- mune diseases, and other disorders. The classification of CD4 H11001 class II–restricted cells as T H cells and CD8 H11001 class I–restricted cells as T C cells is not ab- solute. Some CD4 H11001 cells can act as killer cells. Also, some T C cells have been shown to secrete a variety of cytokines and ex- ert an effect on other cells comparable to that exerted by T H cells. The distinction between T H and T C cells, then, is not al- ways clear; there can be ambiguous functional activities. However, because these ambiguities are the exception and not the rule, the generalization of T helper (T H ) cells as being CD4 H11001 and class-II restricted and of T cytotoxic cells (T C ) as being CD8 H11001 and class-I restricted is assumed throughout this text, unless otherwise specified. T H cells are activated by recognition of an antigen–class II MHC complex on an antigen-presenting cell. After activa- tion, the T H cell begins to divide and gives rise to a clone of effector cells, each specific for the same antigen–class II MHC complex. These T H cells secrete various cytokines, which play a central role in the activation of B cells, T cells, and other cells that participate in the immune response. Changes in the pattern of cytokines produced by T H cells can change the type of immune response that develops among 8536d_ch02_024-056 8/5/02 4:02 PM Page 37 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: 38 PART I Introduction other leukocytes. The T H 1 response produces a cytokine profile that supports inflammation and activates mainly cer- tain T cells and macrophages, whereas the T H 2 response ac- tivates mainly B cells and immune responses that depend upon antibodies. T C cells are activated when they interact with an antigen–class I MHC complex on the surface of an altered self-cell (e.g., a virus-infected cell or a tumor cell) in the presence of appropriate cytokines. This activation, which results in proliferation, causes the T C cell to differentiate into an effector cell called a cytotoxic T lymphocyte (CTL). In contrast to T H cells, most CTLs secrete few cytokines. In- stead, CTLs acquire the ability to recognize and eliminate al- tered self-cells. Another subpopulation of T lymphocytes—called T sup- pressor (T S ) cells—has been postulated. It is clear that some T cells help to suppress the humoral and the cell-mediated branches of the immune system, but the actual isolation and cloning of normal T S cells is a matter of controversy and dis- pute among immunologists. For this reason, it is uncertain whether T S cells do indeed constitute a separate functional subpopulation of T cells. Some immunologists believe that the suppression mediated by T cells observed in some sys- tems is simply the consequence of activities of T H or T C sub- populations whose end results are suppressive. NATURAL KILLER CELLS The natural killer cell was first described in 1976, when it was shown that the body contains a small population of large, granular lymphocytes that display cytotoxic activity against a wide range of tumor cells in the absence of any previous im- munization with the tumor. NK cells were subsequently shown to play an important role in host defense both against tumor cells and against cells infected with some, though not all, viruses. These cells, which constitute 5%–10% of lym- phocytes in human peripheral blood, do not express the membrane molecules and receptors that distinguish T- and B-cell lineages. Although NK cells do not have T-cell recep- tors or immunoglobulin incorporated in their plasma mem- branes, they can recognize potential target cells in two different ways. In some cases, an NK cell employs NK cell re- ceptors to distinguish abnormalities, notably a reduction in the display of class I MHC molecules and the unusual profile of surface antigens displayed by some tumor cells and cells infected by some viruses. Another way in which NK cells rec- ognize potential target cells depends upon the fact that some tumor cells and cells infected by certain viruses display anti- gens against which the immune system has made an anti- body response, so that antitumor or antiviral antibodies are bound to their surfaces. Because NK cells express CD16, a membrane receptor for the carboxyl-terminal end of the IgG molecule, called the Fc region, they can attach to these anti- bodies and subsequently destroy the targeted cells. This is an example of a process known as antibody-dependent cell- mediated cytotoxicity (ADCC). The exact mechanism of NK-cell cytotoxicity, the focus of much current experimental study, is described further in Chapter 14. Several observations suggest that NK cells play an impor- tant role in host defense against tumors. For example, in hu- mans the Chediak-Higashi syndrome—an autosomal recessive disorder—is associated with impairment in neu- trophils, macrophages, and NK cells and an increased inci- dence of lymphomas. Likewise, mice with an autosomal mutation called beige lack NK cells; these mutants are more susceptible than normal mice to tumor growth following in- jection with live tumor cells. There has been growing recognition of a cell type, the NK1-T cell, that has some of the characteristics of both T cells and NK cells. Like T cells, NK1-T cells have T cell recep- tors (TCRs). Unlike most T cells, the TCRs of NK1-T cells in- teract with MHC-like molecules called CD1 rather than with class I or class II MHC molecules. Like NK cells, they have variable levels of CD16 and other receptors typical of NK cells, and they can kill cells. A population of triggered NK1-T cells can rapidly secrete large amounts of the cytokines needed to support antibody production by B cells as well as inflammation and the development and expansion of cyto- toxic T cells. Some immunologists view this cell type as a kind of rapid response system that has evolved to pro- vide early help while conventional T H responses are still developing. Mononuclear Phagocytes The mononuclear phagocytic system consists of monocytes circulating in the blood and macrophages in the tissues (Figure 2-8). During hematopoiesis in the bone marrow, granulocyte-monocyte progenitor cells differentiate into promonocytes, which leave the bone marrow and enter the blood, where they further differentiate into mature monocytes. Monocytes circulate in the bloodstream for about 8 h, during which they enlarge; they then migrate into the tissues and differentiate into specific tissue macrophages or, as discussed later, into dendritic cells. Differentiation of a monocyte into a tissue macrophage involves a number of changes: The cell enlarges five- to ten- fold; its intracellular organelles increase in both number and complexity; and it acquires increased phagocytic ability, pro- duces higher levels of hydrolytic enzymes, and begins to se- crete a variety of soluble factors. Macrophages are dispersed throughout the body. Some take up residence in particular tissues, becoming fixed macrophages, whereas others remain motile and are called free, or wandering, macrophages. Free macrophages travel by amoeboid movement throughout the tissues. Macrophage-like cells serve different functions in different tissues and are named according to their tissue location: a73 Alveolar macrophages in the lung a73 Histiocytes in connective tissues a73 Kupffer cells in the liver a73 Mesangial cells in the kidney 8536d_ch02_024-056 8/5/02 4:02 PM Page 38 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: Cells and Organs of the Immune System CHAPTER 2 39 a73 Microglial cells in the brain a73 Osteoclasts in bone Although normally in a resting state, macrophages are acti- vated by a variety of stimuli in the course of an immune re- sponse. Phagocytosis of particulate antigens serves as an initial activating stimulus. However, macrophage activity can be further enhanced by cytokines secreted by activated T H cells, by mediators of the inflammatory response, and by components of bacterial cell walls. One of the most potent activators of macrophages is interferon gamma (IFN-H9253) se- creted by activated T H cells. Activated macrophages are more effective than resting ones in eliminating potential pathogens, because they exhibit greater phagocytic activity, an increased ability to kill in- gested microbes, increased secretion of inflammatory medi- ators, and an increased ability to activate T cells. In addition, activated macrophages, but not resting ones, secrete various cytotoxic proteins that help them eliminate a broad range of pathogens, including virus-infected cells, tumor cells, and in- tracellular bacteria. Activated macrophages also express higher levels of class II MHC molecules, allowing them to function more effectively as antigen-presenting cells. Thus, macrophages and T H cells facilitate each other’s activation during the immune response. PHAGOCYTOSIS Macrophages are capable of ingesting and digesting exoge- nous antigens, such as whole microorganisms and insoluble particles, and endogenous matter, such as injured or dead host cells, cellular debris, and activated clotting factors. In the first step in phagocytosis, macrophages are attracted by and move toward a variety of substances generated in an immune response; this process is called chemotaxis. The next step in phagocytosis is adherence of the antigen to the macrophage cell membrane. Complex antigens, such as whole bacterial cells or viral particles, tend to adhere well and are readily phagocytosed; isolated proteins and encapsulated bacteria tend to adhere poorly and are less readily phagocytosed. Ad- herence induces membrane protrusions, called pseudopo- dia, to extend around the attached material (Figure 2-9a). Fusion of the pseudopodia encloses the material within a membrane-bounded structure called a phagosome, which then enters the endocytic processing pathway (Figure 2-9b). In this pathway, a phagosome moves toward the cell interior, where it fuses with a lysosome to form a phagolysosome. Lysosomes contain lysozyme and a variety of other hy- drolytic enzymes that digest the ingested material. The di- gested contents of the phagolysosome are then eliminated in a process called exocytosis (see Figure 2-9b). The macrophage membrane has receptors for certain classes of antibody. If an antigen (e.g., a bacterium) is coated with the appropriate antibody, the complex of antigen and antibody binds to antibody receptors on the macrophage membrane more readily than antigen alone and phagocyto- sis is enhanced. In one study, for example, the rate of phago- cytosis of an antigen was 4000-fold higher in the presence of specific antibody to the antigen than in its absence. Thus, an- tibody functions as an opsonin, a molecule that binds to both antigen and macrophage and enhances phagocytosis. The process by which particulate antigens are rendered more susceptible to phagocytosis is called opsonization. ANTIMICROBIAL AND CYTOTOXIC ACTIVITIES A number of antimicrobial and cytotoxic substances pro- duced by activated macrophages can destroy phagocytosed microorganisms (Table 2-6). Many of the mediators of cyto- toxicity listed in Table 2-6 are reactive forms of oxygen. OXYGEN-DEPENDENT KILLING MECHANISMS Activated phagocytes produce a number of reactive oxygen intermedi- ates (ROIs) and reactive nitrogen intermediates that have (a) Monocyte Lysosome Nucleus Phagosome (b) Macrophage Phagosome Phagosome Phagolysosome Lysosome Pseudopodia FIGURE 2-8 Typical morphology of a monocyte and a macrophage. Macrophages are five- to tenfold larger than monocytes and contain more organelles, especially lysosomes. 8536d_ch02_024-056 8/5/02 4:02 PM Page 39 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: 40 PART I Introduction oxide and chloride ions. Hypochlorite, the active agent of household bleach, is toxic to ingested microbes. When macrophages are activated with bacterial cell-wall compo- nents such as lipopolysaccharide (LPS) or, in the case of my- cobacteria, muramyl dipeptide (MDP), together with a T-cell–derived cytokine (IFN-H9253), they begin to express high levels of nitric oxide synthetase (NOS), an enzyme that oxi- dizes L-arginine to yield L-citrulline and nitric oxide (NO), a gas: L-arginine H11001 O 2 H11001NADPH ?→ NO H11001 L-citrulline H11001 NADP Nitric oxide has potent antimicrobial activity; it also can combine with the superoxide anion to yield even more po- tent antimicrobial substances. Recent evidence suggests that much of the antimicrobial activity of macrophages against bacteria, fungi, parasitic worms, and protozoa is due to nitric oxide and substances derived from it. OXYGEN-INDEPENDENT KILLING MECHANISMS Acti- vated macrophages also synthesize lysozyme and various hy- drolytic enzymes whose degradative activities do not require oxygen. In addition, activated macrophages produce a group of antimicrobial and cytotoxic peptides, commonly known as defensins. These molecules are cysteine-rich cationic pep- tides containing 29–35 amino-acid residues. Each peptide, which contains six invariant cysteines, forms a circular mole- cule that is stabilized by intramolecular disulfide bonds. These circularized defensin peptides have been shown to form ion-permeable channels in bacterial cell membranes. Defensins can kill a variety of bacteria, including Staphylo- coccus aureus, Streptococcus pneumoniae, Escherichia coli, potent antimicrobial activity. During phagocytosis, a meta- bolic process known as the respiratory burst occurs in acti- vated macrophages. This process results in the activation of a membrane-bound oxidase that catalyzes the reduction of oxygen to superoxide anion, a reactive oxygen intermediate that is extremely toxic to ingested microorganisms. The su- peroxide anion also generates other powerful oxidizing agents, including hydroxyl radicals and hydrogen peroxide. As the lysosome fuses with the phagosome, the activity of myeloperoxidase produces hypochlorite from hydrogen per- FIGURE 2-9 Macrophages can ingest and degrade particulate antigens, including bacteria. (a) Scanning electron micrograph of a macrophage. Note the long pseudopodia extending toward and mak- ing contact with bacterial cells, an early step in phagocytosis. (b) Phagocytosis and processing of exogenous antigen by macrophages. Pseudopodia Lysosome Phagolysosome Class II MHC Bacteria Phagosome Exocytosed degraded material Antigenic peptide/class II MHC (b)(a) Most of the products resulting from digestion of ingested material are exocytosed, but some peptide products may interact with class II MHC molecules, forming complexes that move to the cell surface, where they are presented to T H cells. [Photograph by L. Nilsson, ? Boehringer Ingelheim International GmbH.] TABLE 2-6 Mediators of antimicrobial and cytotoxic activity of macrophages and neutrophils Oxygen-dependent killing Oxygen-independent killing Reactive oxygen intermediates Defensins O ? 2 H11002 (superoxide anion) Tumor necrosis factor H9251 OH ? (hydroxyl radicals) (macrophage only) H 2 O 2 (hydrogen peroxide) Lysozyme ClO H11002 (hypochlorite anion) Hydrolytic enzymes Reactive nitrogen intermediates NO (nitric oxide) NO 2 (nitrogen dioxide) HNO 2 (nitrous acid) Others NH 2 CL (monochloramine) 8536d_ch02_024-056 8/5/02 4:02 PM Page 40 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: Cells and Organs of the Immune System CHAPTER 2 41 Pseudomonas aeruginosa, and Haemophilus influenzae. Acti- vated macrophages also secrete tumor necrosis factor H9251 (TNF-H9251), a cytokine that has a variety of effects and is cyto- toxic for some tumor cells. ANTIGEN PROCESSING AND PRESENTATION Although most of the antigen ingested by macrophages is de- graded and eliminated, experiments with radiolabeled anti- gens have demonstrated the presence of antigen peptides on the macrophage membrane. As depicted in Figure 2-9b, phagocytosed antigen is digested within the endocytic pro- cessing pathway into peptides that associate with class II MHC molecules; these peptide–class II MHC complexes then move to the macrophage membrane. Activation of macrophages induces increased expression of both class II MHC molecules and the co-stimulatory B7 family of mem- brane molecules, thereby rendering the macrophages more effective in activating T H cells. This processing and presenta- tion of antigen, examined in detail in Chapter 7, are critical to T H -cell activation, a central event in the development of both humoral and cell-mediated immune responses. SECRETION OF FACTORS A number of important proteins central to development of immune responses are secreted by activated macrophages (Table 2-7). These include a collection of cytokines, such as interleukin 1 (IL-1), TNF-H9251 and interleukin 6 (IL-6), that promote inflammatory responses. Typically, each of these agents has a variety of effects. For example, IL-1 activates lymphocytes; and IL-1, IL-6, and TNF-H9251promote fever by af- fecting the thermoregulatory center in the hypothalamus. Activated macrophages secrete a variety of factors in- volved in the development of an inflammatory response. The complement proteins are a group of proteins that assist in eliminating foreign pathogens and in promoting the ensuing inflammatory reaction. The major site of synthesis of com- plement proteins is the liver, although these proteins are also produced in macrophages. The hydrolytic enzymes con- tained within the lysosomes of macrophages also can be se- creted when the cells are activated. The buildup of these enzymes within the tissues contributes to the inflammatory response and can, in some cases, contribute to extensive tis- sue damage. Activated macrophages also secrete soluble fac- tors, such as TNF-H9251, that can kill a variety of cells. The secretion of these cytotoxic factors has been shown to con- tribute to tumor destruction by macrophages. Finally, as mentioned earlier, activated macrophages secrete a number of cytokines that stimulate inducible hematopoiesis. Granulocytic Cells The granulocytes are classified as neutrophils, eosinophils, or basophils on the basis of cellular morphology and cyto- plasmic staining characteristics (Figure 2-10). The neu- trophil has a multilobed nucleus and a granulated cytoplasm that stains with both acid and basic dyes; it is often called a polymorphonuclear leukocyte (PMN) for its multilobed nu- cleus. The eosinophil has a bilobed nucleus and a granulated cytoplasm that stains with the acid dye eosin red (hence its name). The basophil has a lobed nucleus and heavily granu- lated cytoplasm that stains with the basic dye methylene blue. Both neutrophils and eosinophils are phagocytic, whereas basophils are not. Neutrophils, which constitute 50%–70% of the circulating white blood cells, are much more numer- ous than eosinophils (1%–3%) or basophils (H110211%). NEUTROPHILS Neutrophils are produced by hematopoiesis in the bone mar- row. They are released into the peripheral blood and circulate for 7–10 h before migrating into the tissues, where they have a life span of only a few days. In response to many types of in- fections, the bone marrow releases more than the usual num- ber of neutrophils and these cells generally are the first to arrive at a site of inflammation. The resulting transient in- crease in the number of circulating neutrophils, called leuko- cytosis, is used medically as an indication of infection. Movement of circulating neutrophils into tissues, called extravasation, takes several steps: the cell first adheres to the vascular endothelium, then penetrates the gap between adjacent endothelial cells lining the vessel wall, and finally penetrates the vascular basement membrane, moving out into the tissue spaces. (This process is described in detail in Chapter 15.) A number of substances generated in an inflam- matory reaction serve as chemotactic factors that promote accumulation of neutrophils at an inflammatory site. Among these chemotactic factors are some of the complement TABLE 2-7 Some factors secreted by activated macrophages Factor Function Interleukin 1 (IL-1) Promotes inflammatory responses and fever Interleukin 6 (IL-6) Promote innate immunity and TNF-H9251 elimination of pathogens Complement proteins Promote inflammatory response and elimination of pathogens Hydrolytic enzymes Promote inflammatory response Interferon alpha Activates cellular genes, resulting (IFN-H9251) in the production of proteins that confer an antiviral state on the cell Tumor necrosis factor Kills tumor cells (TNF-H9251) GM-CSF G-CSF Promote inducible hematopoiesis M-CSF ? ? ? ? ? ? ? ? 8536d_ch02_024-056 8/5/02 4:02 PM Page 41 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: 42 PART I Introduction components, components of the blood-clotting system, and sev- eral cytokines secreted by activated T H cells and macrophages. Like macrophages, neutrophils are active phagocytic cells. Phagocytosis by neutrophils is similar to that described for macrophages, except that the lytic enzymes and bactericidal substances in neutrophils are contained within primary and secondary granules (see Figure 2-10a). The larger, denser pri- mary granules are a type of lysosome containing peroxidase, lysozyme, and various hydrolytic enzymes. The smaller sec- ondary granules contain collagenase, lactoferrin, and lyso- zyme. Both primary and secondary granules fuse with phagosomes, whose contents are then digested and elimi- nated much as they are in macrophages. Neutrophils also employ both oxygen-dependent and oxygen-independent pathways to generate antimicrobial substances. Neutrophils are in fact much more likely than macrophages to kill ingested microorganisms. Neutrophils exhibit a larger respiratory burst than macrophages and con- sequently are able to generate more reactive oxygen interme- diates and reactive nitrogen intermediates (see Table 2-6). In addition, neutrophils express higher levels of defensins than macrophages do. EOSINOPHILS Eosinophils, like neutrophils, are motile phagocytic cells that can migrate from the blood into the tissue spaces. Their phagocytic role is significantly less important than that of neutrophils, and it is thought that they play a role in the de- fense against parasitic organisms (see Chapter 17). The se- creted contents of eosinophilic granules may damage the parasite membrane. BASOPHILS Basophils are nonphagocytic granulocytes that function by releasing pharmacologically active substances from their cy- toplasmic granules. These substances play a major role in cer- tain allergic responses. MAST CELLS Mast-cell precursors, which are formed in the bone marrow by hematopoiesis, are released into the blood as undifferenti- ated cells; they do not differentiate until they leave the blood and enter the tissues. Mast cells can be found in a wide vari- ety of tissues, including the skin, connective tissues of various organs, and mucosal epithelial tissue of the respiratory, geni- tourinary, and digestive tracts. Like circulating basophils, these cells have large numbers of cytoplasmic granules that contain histamine and other pharmacologically active sub- stances. Mast cells, together with blood basophils, play an im- portant role in the development of allergies. DENDRITIC CELLS The dendritic cell (DC) acquired its name because it is cov- ered with long membrane extensions that resemble the den- drites of nerve cells. Dendritic cells can be difficult to isolate because the conventional procedures for cell isolation tend to damage their long extensions. The development of isolation techniques that employ enzymes and gentler dispersion has facilitated isolation of these cells for study in vitro. There are many types of dendritic cells, although most mature den- dritic cells have the same major function, the presentation of antigen to T H cells. Four types of dendritic cells are known: Langerhans cells, interstitial dendritic cells, myeloid cells, and lymphoid dendritic cells. Each arises from hematopoi- etic stem cells via different pathways and in different loca- tions. Figure 2-11 shows that they descend through both the myeloid and lymphoid lineages. Despite their differences, Phagosome Multilobed nucleus Primary azurophilic granule Secondary granule Glycogen (a) Neutrophil (b) Eosinophil Crystalloid granule (c) Basophil Glycogen Granule FIGURE 2-10 Drawings showing typical morphology of granulo- cytes. Note differences in the shape of the nucleus and in the num- ber and shape of cytoplasmic granules. 8536d_ch02_024-056 8/5/02 4:02 PM Page 42 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: Cells and Organs of the Immune System CHAPTER 2 43 they all constitutively express high levels of both class II MHC molecules and members of the co-stimulatory B7 fam- ily. For this reason, they are more potent antigen-presenting cells than macrophages and B cells, both of which need to be activated before they can function as antigen-presenting cells (APCs). Immature or precursor forms of each of these types of dendritic cells acquire antigen by phagocytosis or endocy- tosis; the antigen is processed, and mature dendritic cells pre- sent it to T H cells. Following microbial invasion or during inflammation, mature and immature forms of Langerhans cells and interstitial dendritic cells migrate into draining lymph nodes, where they make the critical presentation of antigen to T H cells that is required for the initiation of re- sponses by those key cells. Another type of dendritic cell, the follicular dendritic cell (Figure 2-12), does not arise in bone marrow and has a dif- ferent function from the antigen-presenting dendritic cells described above. Follicular dendritic cells do not express class II MHC molecules and therefore do not function as antigen- presenting cells for T H -cell activation. These dendritic cells were named for their exclusive location in organized struc- tures of the lymph node called lymph follicles, which are rich in B cells. Although they do not express class II molecules, follicular dendritic cells express high levels of membrane re- ceptors for antibody, which allows the binding of antigen-an- tibody complexes. The interaction of B cells with this bound antigen can have important effects on B cell responses. Organs of the Immune System A number of morphologically and functionally diverse or- gans and tissues have various functions in the development of immune responses. These can be distinguished by func- tion as the primary and secondary lymphoid organs (Fig- ure 2-13). The thymus and bone marrow are the primary (or central) lymphoid organs, where maturation of lymphocytes takes place. The lymph nodes, spleen, and various mucosal- associated lymphoid tissues (MALT) such as gut-associated lymphoid tissue (GALT) are the secondary (or peripheral) lymphoid organs, which trap antigen and provide sites for mature lymphocytes to interact with that antigen. In addi- tion, tertiary lymphoid tissues, which normally contain fewer lymphoid cells than secondary lymphoid organs, can import lymphoid cells during an inflammatory response. Most prominent of these are cutaneous-associated lymphoid tissues. Once mature lymphocytes have been generated in the primary lymphoid organs, they circulate in the blood and lymphatic system, a network of vessels that collect fluid that has escaped into the tissues from capillaries of the circulatory system and ultimately return it to the blood. Primary Lymphoid Organs Immature lymphocytes generated in hematopoiesis mature and become committed to a particular antigenic specificity within the primary lymphoid organs. Only after a lympho- FIGURE 2-12 Scanning electron micrograph of follicular dendritic cells showing long, beaded dendrites. The beads are coated with anti- gen-antibody complexes. The dendrites emanate from the cell body. [From A. K. Szakal et al., 1985, J. Immunol. 134:1353;? 1996 by American Association of Immunologists, reprinted with permission.] FIGURE 2-11 Dendritic cells arise from both the myeloid and lym- phoid lineages. The myeloid pathway that gives rise to the mono- cyte/macrophage cell type also gives rise to dendritic cells. Some dendritic cells also arise from the lymphoid lineage. These consider- ations do not apply to follicular dendritic cells, which are not derived from bone marrow. Common myeloid progenitor Common lymphoid progenitor Hematopoietic stem cell Langerhans cell Interstitial dendritic cell Myeloid dendritic cell Lymphoid dendritic cell Monocyte Go to www.whfreeman.com/immunology Animation Cells and Organs of the Immune System 8536d_ch02_024-056 9/6/02 9:00 PM Page 43 mac85 Mac 85:365_smm:Goldsby et al. / Immunology 5e: 44 PART I Introduction cyte has matured within a primary lymphoid organ is the cell immunocompetent (capable of mounting an immune re- sponse). T cells arise in the thymus, and in many mammals—humans and mice for example—B cells origi- nate in bone marrow. THYMUS The thymus is the site of T-cell development and maturation. It is a flat, bilobed organ situated above the heart. Each lobe is surrounded by a capsule and is divided into lobules, which are separated from each other by strands of connective tissue called trabeculae. Each lobule is organized into two compart- ments: the outer compartment, or cortex, is densely packed with immature T cells, called thymocytes, whereas the inner compartment, or medulla, is sparsely populated with thymo- cytes. Both the cortex and medulla of the thymus are criss- crossed by a three-dimensional stromal-cell network com- posed of epithelial cells, dendritic cells, and macrophages, which make up the framework of the organ and contribute to the growth and maturation of thymocytes. Many of these stromal cells interact physically with the developing thymo- cytes (Figure 2-14). Some thymic epithelial cells in the outer cortex, called nurse cells, have long membrane extensions that surround as many as 50 thymocytes, forming large mul- ticellular complexes. Other cortical epithelial cells have long interconnecting cytoplasmic extensions that form a network and have been shown to interact with numerous thymocytes as they traverse the cortex. The function of the thymus is to generate and select a repertoire of T cells that will protect the body from infection. As thymocytes develop, an enormous diversity of T-cell re- ceptors is generated by a random process (see Chapter 9) that produces some T cells with receptors capable of recognizing antigen-MHC complexes. However, most of the T-cell recep- tors produced by this random process are incapable of recog- nizing antigen-MHC complexes and a small portion react with combinations of self antigen-MHC complexes. Using mechanisms that are discussed in Chapter 10, the thymus in- duces the death of those T cells that cannot recognize anti- gen-MHC complexes and those that react with self-antigen– MHC and pose a danger of causing autoimmune disease. More than 95% of all thymocytes die by apoptosis in the thy- mus without ever reaching maturity. THE THYMUS AND IMMUNE FUNCTION The role of the thymus in immune function can be studied in mice by exam- ining the effects of neonatal thymectomy, a procedure in which the thymus is surgically removed from newborn mice. These thymectomized mice show a dramatic decrease in cir- culating lymphocytes of the T-cell lineage and an absence of cell-mediated immunity. Other evidence of the importance of the thymus comes from studies of a congenital birth defect in humans (DiGeorge’s syndrome) and in certain mice (nude mice) in which the thymus fails to develop. In both cases, there is an absence of circulating T cells and of cell-me- diated immunity and an increase in infectious disease. Aging is accompanied by a decline in thymic function. This decline may play some role in the decline in immune function during aging in humans and mice. The thymus reaches its maximal size at puberty and then atrophies, with a significant decrease in both cortical and medullary cells and Adenoids Tonsil Thoracic duct Left subclavian vein Lymph nodes Spleen Peyer's patches Small intestine Bone marrow Appendix Large intestine Thymus Right lymphatic duct Tissue lymphatics FIGURE 2-13 The human lymphoid system. The primary organs (bone marrow and thymus) are shown in red; secondary organs and tissues, in blue. These structurally and functionally diverse lymphoid organs and tissues are interconnected by the blood vessels (not shown) and lymphatic vessels (purple) through which lymphocytes circulate. Only one bone is shown, but all major bones contain mar- row and thus are part of the lymphoid system. [Adapted from H. Lodish et al., 1995, Molecular Cell Biology, 3rd ed., Scientific American Books.] 8536d_ch02_024-056 8/5/02 4:02 PM Page 44 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: Cells and Organs of the Immune System CHAPTER 2 45 an increase in the total fat content of the organ. Whereas the average weight of the thymus is 70 g in infants, its age-depen- dent involution leaves an organ with an average weight of only 3 g in the elderly (Figure 2-15). A number of experiments have been designed to look at the effect of age on the immune function of the thymus. In one experiment, the thymus from a 1-day-old or 33-month- old mouse was grafted into thymectomized adults. (For most laboratory mice, 33 months is very old.) Mice receiving the newborn thymus graft showed a significantly larger improve- ment in immune function than mice receiving the 33- month-old thymus. BONE MARROW In humans and mice, bone marrow is the site of B-cell origin and development. Arising from lymphoid progenitors, im- mature B cells proliferate and differentiate within the bone marrow, and stromal cells within the bone marrow interact directly with the B cells and secrete various cytokines that are required for development. Like thymic selection during T- cell maturation, a selection process within the bone marrow eliminates B cells with self-reactive antibody receptors. This process is explained in detail in Chapter 11. Bone marrow is not the site of B-cell development in all species. In birds, a lymphoid organ called the bursa of Fabricius, a lymphoid tissue associated with the gut, is the primary site of B-cell maturation. In mammals such as primates and rodents, there is no bursa and no single counterpart to it as a primary lym- phoid organ. In cattle and sheep, the primary lymphoid tis- sue hosting the maturation, proliferation, and diversification of B cells early in gestation is the fetal spleen. Later in gesta- tion, this function is assumed by a patch of tissue embedded Dividing thymocyte Trabecula Capsule Dead cell Cortical epithelial cell Interdigitating dendritic cell Thymocyte Nurse cell Medulla Cortex Medullary epithelial cell Hassall’s corpusclesMacrophage Blood vessel FIGURE 2-14 Diagrammatic cross section of a portion of the thy- mus, showing several lobules separated by connective tissue strands (trabeculae). The densely populated outer cortex is thought to con- tain many immature thymocytes (blue), which undergo rapid prolif- eration coupled with an enormous rate of cell death. Also present in the outer cortex are thymic nurse cells (gray), which are specialized epithelial cells with long membrane extensions that surround as many as 50 thymocytes. The medulla is sparsely populated and is thought to contain thymocytes that are more mature. During their stay within the thymus, thymocytes interact with various stromal cells, including cortical epithelial cells (light red), medullary epithelial cells (tan), interdigitating dendritic cells (purple), and macrophages (yellow). These cells produce thymic hormones and express high lev- els of class I and class II MHC molecules. Hassalls corpuscles, found in the medulla, contain concentric layers of degenerating ep- ithelial cells. [Adapted, with permission, from W. van Ewijk, 1991, Annu. Rev. Immunol. 9:591,? 1991 by Annual Reviews.] Total thymus weight (g) 50 40 30 20 10 0 10Birth 20 30 40 6050 Age (in years) FIGURE 2-15 Changes in the thymus with age. The thymus de- creases in size and cellularity after puberty. 8536d_ch02_024-056 8/5/02 4:02 PM Page 45 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: 46 PART I Introduction in the wall of the intestine called the ileal Peyer’s patch, which contains a large number (H1102210 10 ) B cells. The rabbit, too, uses gut-associated tissues such as the appendix as primary lym- phoid tissue for important steps in the proliferation and di- versification of B cells. Lymphatic System As blood circulates under pressure, its fluid component (plasma) seeps through the thin wall of the capillaries into the surrounding tissue. Much of this fluid, called interstitial fluid, returns to the blood through the capillary membranes. The remainder of the interstitial fluid, now called lymph, flows from the spaces in connective tissue into a network of tiny open lymphatic capillaries and then into a series of pro- gressively larger collecting vessels called lymphatic vessels (Figure 2-16). The largest lymphatic vessel, the thoracic duct, empties into the left subclavian vein near the heart (see Figure 2-13). In this way, the lymphatic system captures fluid lost from the blood and returns it to the blood, thus ensuring steady-state levels of fluid within the circulatory system. The heart does not pump the lymph through the lymphatic system; instead the flow of lymph is achieved as the lymph vessels are squeezed by movements of the body’s muscles. A series of one-way valves along the lymphatic vessels ensures that lymph flows only in one direction. When a foreign antigen gains entrance to the tissues, it is picked up by the lymphatic system (which drains all the tissues of the body) and is carried to various organized lymphoid tissues such as lymph nodes, which trap the foreign antigen. As lymph passes from the tissues to lym- phatic vessels, it becomes progressively enriched in lympho- cytes. Thus, the lymphatic system also serves as a means of transporting lymphocytes and antigen from the connec- tive tissues to organized lymphoid tissues where the lympho- cytes may interact with the trapped antigen and undergo activation. Secondary Lymphoid Organs Various types of organized lymphoid tissues are located along the vessels of the lymphatic system. Some lymphoid tissue in the lung and lamina propria of the intestinal wall consists of diffuse collections of lymphocytes and macro- phages. Other lymphoid tissue is organized into structures called lymphoid follicles, which consist of aggregates of lym- phoid and nonlymphoid cells surrounded by a network of draining lymphatic capillaries. Until it is activated by anti- gen, a lymphoid follicle—called a primary follicle—com- prises a network of follicular dendritic cells and small resting B cells. After an antigenic challenge, a primary follicle be- comes a larger secondary follicle—a ring of concentrically packed B lymphocytes surrounding a center (the germinal center) in which one finds a focus of proliferating B lympho- cytes and an area that contains nondividing B cells, and some helper T cells interspersed with macrophages and follicular dendritic cells (Figure 2-17). Most antigen-activated B cells divide and differentiate into antibody-producing plasma cells in lymphoid follicles, but only a few B cells in the antigen-activated population find their way into germinal centers. Those that do undergo one or more rounds of cell division, during which the genes that encode their antibodies mutate at an unusually high rate. Following the period of division and mutation, there is a rig- orous selection process in which more than 90% of these B cells die by apoptosis. In general, those B cells producing an- tibodies that bind antigen more strongly have a much better chance of surviving than do their weaker companions. The small number of B cells that survive the germinal center’s rig- orous selection differentiate into plasma cells or memory FIGURE 2-16 Lymphatic vessels. Small lymphatic capillaries open- ing into the tissue spaces pick up interstitial tissue fluid and carry it into progressively larger lymphatic vessels, which carry the fluid, now called lymph, into regional lymph nodes. As lymph leaves the nodes, it is carried through larger efferent lymphatic vessels, which eventu- ally drain into the circulatory system at the thoracic duct or right lymph duct (see Figure 2-13). Lymphatic capillaries Lymphatic vessels Lymphoid follicle Afferent lymphatic vessel Lymph node Secondary follicle Germinal center Efferent lymphatic vessel Tissue space 8536d_ch02_024-056 8/5/02 4:02 PM Page 46 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: Cells and Organs of the Immune System CHAPTER 2 47 cells and emerge. The process of B-cell proliferation, muta- tion, and selection in germinal centers is described more fully in Chapter 11. Lymph nodes and the spleen are the most highly orga- nized of the secondary lymphoid organs; they comprise not only lymphoid follicles, but additional distinct regions of T- cell and B-cell activity, and they are surrounded by a fibrous capsule. Less-organized lymphoid tissue, collectively called mucosal-associated lymphoid tissue (MALT), is found in various body sites. MALT includes Peyer’s patches (in the small intestine), the tonsils, and the appendix, as well as nu- merous lymphoid follicles within the lamina propria of the intestines and in the mucous membranes lining the upper airways, bronchi, and genital tract. LYMPH NODES Lymph nodes are the sites where immune responses are mounted to antigens in lymph. They are encapsulated bean- shaped structures containing a reticular network packed with lymphocytes, macrophages, and dendritic cells. Clus- tered at junctions of the lymphatic vessels, lymph nodes are the first organized lymphoid structure to encounter antigens that enter the tissue spaces. As lymph percolates through a node, any particulate antigen that is brought in with the lymph will be trapped by the cellular network of phagocytic cells and dendritic cells (follicular and interdigitating). The overall architecture of a lymph node supports an ideal mi- croenvironment for lymphocytes to effectively encounter and respond to trapped antigens. Morphologically, a lymph node can be divided into three roughly concentric regions: the cortex, the paracortex, and the medulla, each of which supports a distinct microenviron- ment (Figure 2-18). The outermost layer, the cortex, contains lymphocytes (mostly B cells), macro-phages, and follicular dendritic cells arranged in primary follicles. After antigenic challenge, the primary follicles enlarge into secondary folli- cles, each containing a germinal center. In children with B-cell deficiencies, the cortex lacks primary follicles and germinal centers. Beneath the cortex is the paracortex, which is popu- lated largely by T lymphocytes and also contains interdigitat- ing dendritic cells thought to have migrated from tissues to the node. These interdigitating dendritic cells express high levels of class II MHC molecules, which are necessary for pre- senting antigen to T H cells. Lymph nodes taken from neona- tally thymectomized mice have unusually few cells in the paracortical region; the paracortex is therefore sometimes re- ferred to as a thymus-dependent area in contrast to the cor- tex, which is a thymus-independent area. The innermost layer of a lymph node, the medulla, is more sparsely popu- lated with lymphoid-lineage cells; of those present, many are plasma cells actively secreting antibody molecules. As antigen is carried into a regional node by the lymph, it is trapped, processed, and presented together with class II MHC molecules by interdigitating dendritic cells in the para- cortex, resulting in the activation of T H cells. The initial acti- vation of B cells is also thought to take place within the T-cell-rich paracortex. Once activated, T H and B cells form small foci consisting largely of proliferating B cells at the edges of the paracortex. Some B cells within the foci differen- tiate into plasma cells secreting IgM and IgG. These foci reach maximum size within 4–6 days of antigen challenge. Within 4–7 days of antigen challenge, a few B cells and T H cells migrate to the primary follicles of the cortex. It is not known what causes this migration. Within a primary follicle, cellular interactions between follicular dendritic cells, B cells, and T H cells take place, leading to development of a sec- ondary follicle with a central germinal center. Some of the plasma cells generated in the germinal center move to the medullary areas of the lymph node, and many migrate to bone marrow. Afferent lymphatic vessels pierce the capsule of a lymph node at numerous sites and empty lymph into the subcapsu- lar sinus (see Figure 2-18b). Lymph coming from the tissues percolates slowly inward through the cortex, paracortex, and medulla, allowing phagocytic cells and dendritic cells to trap any bacteria or particulate material (e.g., antigen-antibody complexes) carried by the lymph. After infection or the FIGURE 2-17 A secondary lymphoid follicle consisting of a large germinal center (gc) surrounded by a dense mantle (m) of small lym- phocytes. [From W. Bloom and D. W. Fawcett, 1975, Textbook of His- tology, 10th ed., ? 1975 by W. B. Saunders Co.] gc m 8536d_ch02_024-056 8/5/02 4:02 PM Page 47 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: 48 PART I Introduction introduction of other antigens into the body, the lymph leav- ing a node through its single efferent lymphatic vessel is en- riched with antibodies newly secreted by medullary plasma cells and also has a fiftyfold higher concentration of lympho- cytes than the afferent lymph. The increase in lymphocytes in lymph leaving a node is due in part to lymphocyte proliferation within the node in response to antigen. Most of the increase, however, repre- sents blood-borne lymphocytes that migrate into the node by passing between specialized endothelial cells that line the postcapillary venules of the node. Estimates are that 25% of the lymphocytes leaving a lymph node have migrated across this endothelial layer and entered the node from the blood. Because antigenic stimulation within a node can increase this migration tenfold, the concentration of lymphocytes in a node that is actively responding can increase greatly, and the node swells visibly. Factors released in lymph nodes during antigen stimulation are thought to facilitate this increased migration. FIGURE 2-18 Structure of a lymph node. (a) The three layers of a lymph node support distinct microenvironments. (b) The left side depicts the arrangement of reticulum and lymphocytes within the various regions of a lymph node. Macrophages and dendritic cells, which trap antigen, are present in the cortex and paracortex. T H cells are concentrated in the paracortex; B cells are located primarily in the cortex, within follicles and germinal centers. The medulla is popu- Cortex Paracortex Medulla Efferent lymphatic vessel Lymphatic artery Germinal centers Capsule Primary lymphoid follicle (a) (b) Cross section post- capillary venule B lymphocytes B lymphocytes Afferent lymphatic vessels Germinal centers Postcapillary venule Capsule Lymphatic vein lated largely by antibody-producing plasma cells. Lymphocytes circu- lating in the lymph are carried into the node by afferent lymphatic vessels; they either enter the reticular matrix of the node or pass through it and leave by the efferent lymphatic vessel. The right side of (b) depicts the lymphatic artery and vein and the postcapillary venules. Lymphocytes in the circulation can pass into the node from the postcapillary venules by a process called extravasation (inset). 8536d_ch02_024-056 8/5/02 4:02 PM Page 48 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: Cells and Organs of the Immune System CHAPTER 2 49 SPLEEN The spleen plays a major role in mounting immune re- sponses to antigens in the blood stream. It is a large, ovoid secondary lymphoid organ situated high in the left abdomi- nal cavity. While lymph nodes are specialized for trapping antigen from local tissues, the spleen specializes in filtering blood and trapping blood-borne antigens; thus, it can re- spond to systemic infections. Unlike the lymph nodes, the spleen is not supplied by lymphatic vessels. Instead, blood- borne antigens and lymphocytes are carried into the spleen through the splenic artery. Experiments with radioactively labeled lymphocytes show that more recirculating lympho- cytes pass daily through the spleen than through all the lymph nodes combined. The spleen is surrounded by a capsule that extends a num- ber of projections (trabeculae) into the interior to form a compartmentalized structure. The compartments are of two types, the red pulp and white pulp, which are separated by a diffuse marginal zone (Figure 2-19). The splenic red pulp consists of a network of sinusoids populated by macrophages and numerous red blood cells (erythrocytes) and few lym- phocytes; it is the site where old and defective red blood cells are destroyed and removed. Many of the macrophages within the red pulp contain engulfed red blood cells or iron pigments from degraded hemoglobin. The splenic white pulp sur- rounds the branches of the splenic artery, forming a periarte- riolar lymphoid sheath (PALS) populated mainly by T lymphocytes. Primary lymphoid follicles are attached to the FIGURE 2-19 Structure of the spleen. (a) The spleen, which is about 5 inches long in adults, is the largest secondary lymphoid or- gan. It is specialized for trapping blood-borne antigens. (b) Diagram- matic cross section of the spleen. The splenic artery pierces the capsule and divides into progressively smaller arterioles, ending in vascular sinusoids that drain back into the splenic vein. The erythro- Capsule Trabecula Vascular sinusoid Red pulp Vein Artery Germinal center White pulp Renal surface Splenic artery Splenic vein Gastric surface Hilum (a) (b) Primary follicle Marginal zone Periarteriolar lymphoid sheath (PALS) cyte-filled red pulp surrounds the sinusoids. The white pulp forms a sleeve, the periarteriolar lymphoid sheath (PALS), around the arteri- oles; this sheath contains numerous T cells. Closely associated with the PALS is the marginal zone, an area rich in B cells that contains lymphoid follicles that can develop into secondary follicles contain- ing germinal centers. 8536d_ch02_024-056 8/7/02 8:25 AM Page 49 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: 50 PART I Introduction PALS. These follicles are rich in B cells and some of them con- tain germinal centers. The marginal zone, located peripheral to the PALS, is populated by lymphocytes and macrophages. Blood-borne antigens and lymphocytes enter the spleen through the splenic artery, which empties into the marginal zone. In the marginal zone, antigen is trapped by interdigi- tating dendritic cells, which carry it to the PALS. Lympho- cytes in the blood also enter sinuses in the marginal zone and migrate to the PALS. The initial activation of B and T cells takes place in the T- cell-rich PALS. Here interdigitating dendritic cells capture antigen and present it combined with class II MHC mole- cules to T H cells. Once activated, these T H cells can then acti- vate B cells. The activated B cells, together with some T H cells, then migrate to primary follicles in the marginal zone. Upon antigenic challenge, these primary follicles develop into char- acteristic secondary follicles containing germinal centers (like those in the lymph nodes), where rapidly dividing B cells (centroblasts) and plasma cells are surrounded by dense clusters of concentrically arranged lymphocytes. The effects of splenectomy on the immune response de- pends on the age at which the spleen is removed. In children, splenectomy often leads to an increased incidence of bacterial sepsis caused primarily by Streptococcus pneumoniae, Neisse- ria meningitidis, and Haemophilus influenzae. Splenectomy in adults has less adverse effects, although it leads to some in- crease in blood-borne bacterial infections (bacteremia). MUCOSAL-ASSOCIATED LYMPHOID TISSUE The mucous membranes lining the digestive, respiratory, and urogenital systems have a combined surface area of about 400 m 2 (nearly the size of a basketball court) and are the ma- jor sites of entry for most pathogens. These vulnerable mem- brane surfaces are defended by a group of organized lymphoid tissues mentioned earlier and known collectively as mucosal-associated lymphoid tissue (MALT). Struc- turally, these tissues range from loose, barely organized clus- ters of lymphoid cells in the lamina propria of intestinal villi to well-organized structures such as the familiar tonsils and appendix, as well as Peyer’s patches, which are found within the submucosal layer of the intestinal lining. The functional importance of MALT in the body’s defense is attested to by its large population of antibody-producing plasma cells, whose number far exceeds that of plasma cells in the spleen, lymph nodes, and bone marrow combined. The tonsils are found in three locations: lingual at the base of the tongue; palatine at the sides of the back of the mouth; and pharyngeal (adenoids) in the roof of the na- sopharynx (Figure 2-20). All three tonsil groups are nodular structures consisting of a meshwork of reticular cells and fibers interspersed with lymphocytes, macrophages, granulo- cytes, and mast cells. The B cells are organized into follicles and germinal centers; the latter are surrounded by regions showing T-cell activity. The tonsils defend against antigens entering through the nasal and oral epithelial routes. The best studied of the mucous membranes is the one that lines the gastrointestinal tract. This tissue, like that of the res- piratory and urogenital tracts, has the capacity to endocytose antigen from the lumen. Immune reactions are initiated against pathogens and antibody can be generated and ex- ported to the lumen to combat the invading organisms. As shown in Figures 2-21 and 2-22, lymphoid cells are found in various regions within this tissue. The outer mucosal epithe- FIGURE 2-20 Three types of tonsils. (a) The position and internal features of the palatine and lingual tonsils; (b) a view of the position of the nasopharyngeal tonsils (adenoids). [Part b adapted from (a) Palatine tonsil Lingual tonsils Lingual tonsils Tongue Lymphoid tissue Crypt Cross section of palatine tonsil Cross section of tongue at lingual tonsil Papilla with taste buds Pharyngeal tonsil (adenoid) (b) J. Klein, 1982, Immunology, The Science of Self-Nonself Discrimina- tion, ? 1982 by John Wiley and Sons, Inc.] 8536d_ch02_024-056 8/5/02 4:02 PM Page 50 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: Cells and Organs of the Immune System CHAPTER 2 51 lial layer contains so-called intraepithelial lymphocytes (IELs). Many of these lymphocytes are T cells that express unusual receptors (H9253H9254T-cell receptors, or H9253H9254 TCRs), which exhibit limited diversity for antigen. Although this popula- tion of T cells is well situated to encounter antigens that en- ter through the intestinal mucous epithelium, their actual function remains largely unknown The lamina propria, which lies under the epithelial layer, contains large numbers of B cells, plasma cells, activated T H cells, and macrophages in loose clusters. Histologic sections have revealed more than 15,000 lymphoid follicles within the intestinal lamina pro- pria of a healthy child. The submucosal layer beneath the lamina propria contains Peyer’s patches, nodules of 30–40 lymphoid follicles. Like lymphoid follicles in other sites, those that compose Peyer’s patches can develop into sec- ondary follicles with germinal centers. The epithelial cells of mucous membranes play an impor- tant role in promoting the immune response by delivering small samples of foreign antigen from the lumina of the res- piratory, digestive, and urogenital tracts to the underlying mucosal-associated lymphoid tissue. This antigen transport is carried out by specialized M cells. The structure of the M cell is striking: these are flattened epithelial cells lacking the microvilli that characterize the rest of the mucous epithe- lium. In addition, M cells have a deep invagination, or pocket, in the basolateral plasma membrane; this pocket is filled with a cluster of B cells, T cells, and macrophages (Fig- ure 2-22a). Luminal antigens are endocytosed into vesicles that are transported from the luminal membrane to the un- derlying pocket membrane. The vesicles then fuse with the pocket membrane, delivering the potentially response-acti- vating antigens to the clusters of lymphocytes contained within the pocket. M cells are located in so-called inductive sites—small re- gions of a mucous membrane that lie over organized lym- phoid follicles (Figure 2-22b). Antigens transported across the mucous membrane by M cells can activate B cells within FIGURE 2-21 Cross-sectional diagram of the mucous membrane lining the intestine showing a nodule of lymphoid follicles that con- stitutes a Peyer’s patch in the submucosa. The intestinal lamina pro- pria contains loose clusters of lymphoid cells and diffuse follicles. FIGURE 2-22 Structure of M cells and production of IgA at induc- tive sites. (a) M cells, located in mucous membranes, endocytose antigen from the lumen of the digestive, respiratory, and urogenital tracts. The antigen is transported across the cell and released into the large basolateral pocket. (b) Antigen transported across the epithelial layer by M cells at an inductive site activates B cells in the underlying M cell VilliFollicle Primary follicle Peyer’s patch Germinal center Muscle layer SubmucosaLamina propria Inductive site Intestinal lumen Organized lymphoid follicle M cell Mucosal epithelium Lamina propria Antigen Plasma cell Lumen IgA Intraepithelial lymphocyte IgA (b) Pocket B cells T H cell M cell Antigen Macrophage (a) lymphoid follicles. The activated B cells differentiate into IgA-pro- ducing plasma cells, which migrate along the submucosa. The outer mucosal epithelial layer contains intraepithelial lymphocytes, of which many are CD8 H11001 T cells that express H9253H9254 TCRs with limited re- ceptor diversity for antigen. 8536d_ch02_024-056 8/5/02 4:02 PM Page 51 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: 52 PART I Introduction these lymphoid follicles. The activated B cells differentiate into plasma cells, which leave the follicles and secrete the IgA class of antibodies. These antibodies then are transported across the epithelial cells and released as secretory IgA into the lumen, where they can interact with antigens. As described in Chapter 1, mucous membranes are an ef- fective barrier to the entrance of most pathogens, which thereby contributes to nonspecific immunity. One reason for this is that the mucosal epithelial cells are cemented to one another by tight junctions that make it difficult for patho- gens to penetrate. Interestingly, some enteric pathogens, in- cluding both bacteria and viruses, have exploited the M cell as an entry route through the mucous-membrane barrier. In some cases, the pathogen is internalized by the M cell and transported into the pocket. In other cases, the pathogen binds to the M cell and disrupts the cell, thus allowing entry of the pathogen. Among the pathogens that use M cells in these ways are several invasive Salmonella species, Vibrio cholerae, and the polio virus. Cutaneous-Associated Lymphoid Tissue The skin is an important anatomic barrier to the external en- vironment, and its large surface area makes this tissue impor- tant in nonspecific (innate) defenses. The epidermal (outer) layer of the skin is composed largely of specialized epithelial cells called keratinocytes. These cells secrete a number of cy- tokines that may function to induce a local inflammatory re- action. In addition, keratinocytes can be induced to express class II MHC molecules and may function as antigen-present- ing cells. Scattered among the epithelial-cell matrix of the epi- dermis are Langerhans cells, a type of dendritic cell, which internalize antigen by phagocytosis or endocytosis. The Langerhans cells then migrate from the epidermis to regional lymph nodes, where they differentiate into interdigitating dendritic cells. These cells express high levels of class II MHC molecules and function as potent activators of naive T H cells. The epidermis also contains so-called intraepidermal lym- phocytes. These are similar to the intraepithelial lymphocytes of MALT in that most of them are CD8 H11001 T cells, many of which express H9253H9254 T-cell receptors, which have limited diver- sity for antigen. These intraepidermal T cells are well situated to encounter antigens that enter through the skin and some immunologists believe that they may play a role in combat- ing antigens that enter through the skin. The underlying der- mal layer of the skin contains scattered CD4 H11001 and CD8 H11001 T cells and macrophages. Most of these dermal T cells were ei- ther previously activated cells or are memory cells. Systemic Function of the Immune System The many different cells, organs, and tissues of the immune system are dispersed throughout the body, yet the various components communicate and collaborate to produce an ef- fective response to an infection. An infection that begins in one area of the body initiates processes that eventually in- volve cells, organs, and tissues distant from the site of pathogen invasion. Consider what happens when the skin is broken, allowing bacteria to enter the body and initiate infection. The tissue damage associated with the injury and infec- tion results in an inflammatory response that causes in- creased blood flow, vasodilation, and an increase in capillary permeability. Chemotactic signals are generated that can cause phagocytes and lymphocytes to leave the blood stream and enter the affected area. Factors generated during these early stages of the infection stimulate the capacity of the adaptive immune system to respond. Langerhans cells (den- dritic cells found throughout the epithelial layers of the skin and the respiratory, gastrointestinal, urinary, and genital tracts) can capture antigens from invading pathogens and migrate into a nearby lymphatic vessel, where the flow of lymph carries them to nearby lymph nodes. In the lymph nodes these class II MHC–bearing cells can become mem- bers of the interdigitating dendritic-cell population and ini- tiate adaptive immune responses by presenting antigen to T H cells. The recognition of antigen by T H cells can have impor- tant consequences, including the activation and proliferation of T H cells within the node as the T H cells recognize the anti- gen, and the secretion by the activated T cells of factors that support T-cell–dependent antibody production by B cells that may already have been activated by antigen delivered to the lymph node by lymph. The antigen-stimulated T H cells also release chemotactic factors that cause lymphocytes to leave the blood circulation and enter the lymph node through the endothelium of the postcapillary venules. Lym- phocytes that respond to the antigen are retained in the lymph node for 48 hours or so as they undergo activation and proliferation before their release via the node’s efferent lymphatic vessel. Once in the lymph, the newly released activated lympho- cytes can enter the bloodstream via the subclavian vein. Eventually, the circulation carries them to blood vessels near the site of the infection, where the inflammatory process makes the vascular endothelium of the nearby blood vessels more adherent for activated T cells and other leukocytes (see Chapter 15). Chemotactic factors that attract lymphocytes, macrophages, and neutrophils are also generated during the inflammatory process, promoting leukocyte adherence to nearby vascular epithelium and leading leukocytes to the site of the infection. Later in the course of the response, pathogen-specific antibodies produced in the node are also carried to the bloodstream. Inflammation aids the delivery of the anti-pathogen antibody by promoting increased vascular permeability, which increases the flow of antibody-contain- ing plasma from the blood circulation to inflamed tissue. The result of this network of interactions among diffusible mole- cules, cells, organs, the lymphatic system, and the circulatory system is an effective and focused immune response to an infection. 8536d_ch02_024-056 8/5/02 4:02 PM Page 52 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: Cells and Organs of the Immune System CHAPTER 2 53 Lymphoid Cells and Organs— Evolutionary Comparisons While innate systems of immunity are seen in invertebrates and even in plants, the evolution of lymphoid cells and or- gans evolved only in the phylum Vertebrata. Consequently, adaptive immunity, which is mediated by antibodies and T cells, is only seen in this phylum. However, as shown in Fig- ure 2-23, the kinds of lymphoid tissues seen in different or- ders of vertebrates differ. As one considers the spectrum from the earliest verte- brates, the jawless fishes (Agnatha), to the birds and mam- mals, evolution has added organs and tissues with immune GALT Thymus Vertebrata GnathostomataAgnatha Spleen Bone marrow Lymph nodes Germinal centers MammaliaAvesTeleostei Lamprey Trout Frog Chicken Lymph nodes Lymph nodes Peyer's patch GALT Spleen Bone marrow Mouse Reptilia Amphibia Anura Osteichthyes Thymus Thymus Thymus Thymus Kidney GALT GALT GALT GALT Spleen Spleen Bone marrow Bursa Bone marrow Spleen FIGURE 2-23 Evolutionary distribution of lymphoid tissues. The presence and location of lymphoid tissues in several major orders of vertebrates are shown. Although they are not shown in the diagram, cartilaginous fish such as sharks and rays have GALT, thymus, and a spleen. Reptiles also have GALT, thymus, and spleen and they also may have lymph nodes that participate in immunological reactions. Whether bone marrow is involved in the generation of lymphocytes in reptiles is under investigation. [Adapted from Dupasquier and M. Flajnik, 1999.InFundamental Immunology 4th ed., W. E. Paul, ed., Lippincott-Raven, Philadelphia.] 8536d_ch02_024-056 8/5/02 4:02 PM Page 53 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: 54 PART I Introduction functions but has tended to retain those evolved by earlier or- ders. While all have gut-associated lymphoid tissue (GALT) and most have some version of a spleen and thymus, not all have blood-cell-forming bone marrow or lymph nodes, and the ability to form germinal centers is not shared by all. The differences seen at the level of organs and tissues are also re- flected at the cellular level. Lymphocytes that express anti- gen-specific receptors on their surfaces are necessary to mount an adaptive immune response. So far, it has not been possible to demonstrate the presence of T or B lymphocytes in the jawless fishes, and attempts to demonstrate an adaptive immune response in lampreys and hagfish, members of the order Agnatha, have failed. In fact, only jawed vertebrates (Gnathosomata), of which the cartilaginous fish (sharks, rays) are the earliest example, have B and T lymphocytes and support adaptive immune responses. SUMMARY a73 The cells that participate in the immune response are white blood cells, or leukocytes. The lymphocyte is the only cell to possess the immunologic attributes of specificity, diver- sity, memory, and self/nonself recognition. a73 Many of the body’s cells, tissues, and organs arise from the progeny of different stem-cell populations. The division of a stem cell can result in the production of another stem cell and a differentiated cell of a specific type or group. a73 All leukocytes develop from a common multipotent hematopoietic stem cell during hematopoiesis. Various hematopoietic growth factors (cytokines) induce prolifer- ation and differentiation of the different blood cells. The differentiation of stem cells into different cell types re- quires the expression of different lineage-determining genes. A number of transcription factors play important roles in this regard. a73 Hematopoiesis is closely regulated to assure steady-state levels of each of the different types of blood cell. Cell divi- sion and differentiation of each of the lineages is balanced by programmed cell death. a73 There are three types of lymphocytes: B cells, T cells, and natural killer cells (NK cells). NK cells are much less abun- dant than B and T cells, and most lack a receptor that is specific for a particular antigen. However, a subtype of NK cells, NK1-T cells, have both T-cell receptors and many of the markers characteristic of NK cells. The three types of lymphoid cells are best distinguished on the basis of func- tion and the presence of various membrane molecules. a73 Naive B and T lymphocytes (those that have not encoun- tered antigen) are small resting cells in the G 0 phase of the cell cycle. After interacting with antigen, these cells enlarge into lymphoblasts that proliferate and eventually differen- tiate into effector cells and memory cells. a73 Macrophages and neutrophils are specialized for the phagocytosis and degradation of antigens (see Figure 2-9). Phagocytosis is facilitated by opsonins such as antibody, which increase the attachment of antigen to the membrane of the phagocyte. a73 Activated macrophages secrete various factors that regu- late the development of the adaptive immune response and mediate inflammation (see Table 2-7). Macrophages also process and present antigen bound to class II MHC molecules, which can then be recognized by T H cells. a73 Basophils and mast cells are nonphagocytic cells that re- lease a variety of pharmacologically active substances and play important roles in allergic reactions. a73 Dendritic cells capture antigen. With the exception of fol- licular dendritic cells, these cells express high levels of class II MHC molecules. Along with macrophages and B cells, dendritic cells play an important role in T H -cell activation by processing and presenting antigen bound to class II MHC molecules and by providing the required co-stimula- tory signal. Follicular dendritic cells, unlike the others, fa- cilitate B-cell activation but play no role in T-cell activation. a73 The primary lymphoid organs provide sites where lympho- cytes mature and become antigenically committed. T lym- phocytes mature within the thymus, and B lymphocytes arise and mature within the bone marrow of humans, mice, and several other animals, but not all vertebrates. a73 Primary lymphoid organs are also places of selection where many lymphocytes that react with self antigens are eliminated. Furthermore, the thymus eliminates thymo- cytes that would mature into useless T cells because their T-cell receptors are unable to recognize self-MHC. a73 The lymphatic system collects fluid that accumulates in tis- sue spaces and returns this fluid to the circulation via the left subclavian vein. It also delivers antigens to the lymph nodes, which interrupt the course of lymphatic vessels. a73 Secondary lymphoid organs capture antigens and provide sites where lymphocytes become activated by interaction with antigens. Activated lymphocytes undergo clonal pro- liferation and differentiation into effector cells. a73 There are several types of secondary lymphoid tissue: lymph nodes, spleen, the loose clusters of follicles, and Peyer’s patches of the intestine, and cutaneous-associated lymphoid tissue. Lymph nodes trap antigen from lymph, spleen traps blood-borne antigens, intestinal-associated lymphoid tissues (as well as other secondary lymphoid tis- sues) interact with antigens that enter the body from the gastrointestinal tract, and cutaneous-associated lymphoid tissue protects epithelial tissues. a73 An infection that begins in one area of the body eventually involves cells, organs, and tissues that may be distant from the site of pathogen invasion. Antigen from distant sites can arrive at lymph nodes via lymph and dendritic cells, thereby assuring activation of T cells and B cells and release of these cells and their products to the circulation. Inflam- matory processes bring lymphocytes and other leukocytes to the site of infection. Thus, although dispersed through- 8536d_ch02_024-056 8/5/02 4:02 PM Page 54 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: Cells and Organs of the Immune System CHAPTER 2 55 out the body, the components of the immune system com- municate and collaborate to produce an effective response to infection. a73 Vertebrate orders differ greatly in the kinds of lymphoid organs, tissues, and cells they possess. The most primitive vertebrates, the jawless fishes, have only gut-associated lymphoid tissues, lack B and T cells, and cannot mount adaptive immune responses. Jawed vertebrates possess a greater variety of lymphoid tissues, have B and T cells, and display adaptive immunity. References Appelbaum, F. R. 1996. Hematopoietic stem cell transplanta- tion. In Scientific American Medicine. D. Dale and D. Feder- man, eds. Scientific American Publishers, New York. Banchereau J., F. Briere, C. Caux, J. Davoust, S. Lebecque, Y. J. Liu, B. Pulendran, and K. Palucka. 2000. Immunobiology of dendritic cells. Annu.Rev.Immunology.18:767. Bendelac, A., M. N. Rivera, S-H. Park, and J. H. Roark. 1997. Mouse CD1-specific NK1 T cells: Development, specificity and function. Annu.Rev.Immunol.15:535. Clevers, H. C., and R. Grosschedl. 1996. Transcriptional control of lymphoid development: lessons from gene targeting. Im- munol. Today 17:336. Cory, S. 1995. Regulation of lymphocyte survival by the BCL-2 gene family. Annu.Rev.Immunol.12:513. Ganz, T., and R. I. Lehrer. 1998. Antimicrobial peptides of verte- brates. Curr. Opin. Immunol. 10:41. Liu,Y. J. 2001. Dendritic cell subsets and lineages, and their func- tions in innate and adaptive immunity. Cell 106:259. Melchers, F., and A. Rolink. 1999. B-lymphocyte development and biology. In Fundamental Immunology, 4th ed., W. E. Paul, ed., p. 183. Lippincott-Raven, Philadelphia. Nathan, C., and M. U. Shiloh. 2000. Reactive oxygen and nitro- gen intermediates in the relationship between mammalian hosts and microbial pathogens. Proc. Natl. Acad. Sci. 97:8841. Pedersen, R. A. 1999. Embryonic stem cells for medicine. Sci. Am. 280:68. Osborne, B. A. 1996. Apoptosis and the maintenance of home- ostasis in the immune system. Curr. Opin. Immunol. 8:245. Picker, L. J., and M. H. Siegelman. 1999. Lymphoid tissues and organs. In Fundamental Immunology, 4th ed., W. E. Paul, ed., p. 145. Lippincott-Raven, Philadelphia. Rothenberg, E.V. 2000. Stepwise specification of lymphocyte de- velopmental lineages. Current Opin. Gen. Dev. 10:370. Ward, A. C., D. M. Loeb, A. A. Soede-Bobok, I. P. Touw, and A. D. Friedman. 2000. Regulation of granulopoiesis by transcription factors and cytokine signals. Leukemia 14:973. Weissman, I. L. 2000. Translating stem and progenitor cell biology to the clinic: barriers and opportunities. Science 287:1442. USEFUL WEB SITES http://www.ncbi.nlm.nih.gov/prow The PROW Guides are authoritative short, structured reviews on proteins and protein families that bring together the most relevant information on each molecule into a single docu- ment of standardized format. http://hms.medweb.harvard.edu/nmw/HS_heme/ AtlasTOC.htm This brilliantly illustrated atlas of normal and abnormal blood cells informatively displayed as stained cell smears has been assembled to help train medical students at the Harvard Medical School to recognize and remember cell morphology that is associated with many different pathologies, including leukemias, anemias, and even malarial infections. http://www.nih.gov/news/stemcell/primer.htm This site provides a brief, but informative introduction to stem cells, including their importance and promise as tools for research and therapy. http://www.nih.gov/news/stemcell/scireport.htm A well written and comprehensive presentation of stem cells and their biology is presented in an interesting and well-refer- enced monograph. Study Questions CLINICAL FOCUS QUESTION The T and B cells that differentiate from hematopoietic stem cells recognize as self the bodies in which they differentiate. Suppose a woman donates HSCs to a genetically unrelated man whose hematopoietic system was to- tally destroyed by a combination of radiation and chemother- apy. Suppose further that, although most of the donor HSCs differentiate into hematopoietic cells, some differentiate into cells of the pancreas, liver, and heart. Decide which of the fol- lowing outcomes is likely and justify your choice. a. The T cells from the donor HSCs do not attack the pancre- atic, heart, and liver cells that arose from donor cells, but mount a GVH response against all of the other host cells. b. The T cells from the donor HSCs mount a GVH response against all of the host cells. c. The T cells from the donor HSCs attack the pancreatic, heart, and liver cells that arose from donor cells, but fail to mount a GVH response against all of the other host cells. d. The T cells from the donor HSCs do not attack the pancreatic, heart, and liver cells that arose from donor cells and fail to mount a GVH response against all of the other host cells. 1. Explain why each of the following statements is false. a. All T H cells express CD4 and recognize only antigen asso- ciated with class II MHC molecules. b. The pluripotent stem cell is one of the most abundant cell types in the bone marrow. c. Activation of macrophages increases their expression of class I MHC molecules, making the cells present antigen more effectively. Go to www.whfreeman.com/immunology Self-Test Review and quiz of key terms 8536d_ch02_024-056 9/6/02 9:00 PM Page 55 mac85 Mac 85:365_smm:Goldsby et al. / Immunology 5e: 56 PART I Introduction d. Lymphoid follicles are present only in the spleen and lymph nodes. e. Infection has no influence on the rate of hematopoiesis. f. Follicular dendritic cells can process and present antigen to T lymphocytes. g. All lymphoid cells have antigen-specific receptors on their membrane. h. All vertebrates generate B lymphocytes in bone marrow. i. All vertebrates produce B or T lymphocytes and most produce both. 2. For each of the following situations, indicate which type(s) of lymphocyte(s), if any, would be expected to proliferate rapidly in lymph nodes and where in the nodes they would do so. a. Normal mouse immunized with a soluble protein anti- gen b. Normal mouse with a viral infection c. Neonatally thymectomized mouse immunized with a protein antigen d. Neonatally thymectomized mouse immunized with the thymus-independent antigen bacterial lipopolysaccha- ride (LPS), which does not require the aid of T H cells to activate B cells 3. List the primary lymphoid organs and summarize their functions in the immune response. 4. List the secondary lymphoid organs and summarize their functions in the immune response. 5. What are the two primary characteristics that distinguish hematopoietic stem cells and progenitor cells? 6. What are the two primary roles of the thymus? 7. What do nude mice and humans with DiGeorge’s syndrome have in common? 8. At what age does the thymus reach its maximal size? a. During the first year of life b. Teenage years (puberty) c. Between 40 and 50 years of age d. After 70 years of age 9. Preparations enriched in hematopoietic stem cells are useful for research and clinical practice. In Weissman’s method for enriching hematopoietic stem cells, why is it necessary to use lethally irradiated mice to demonstrate enrichment? 10. What effect does thymectomy have on a neonatal mouse? On an adult mouse? Explain why these effects differ. 11. What effect would removal of the bursa of Fabricius (bur- sectomy) have on chickens? 12. Some microorganisms (e.g., Neisseria gonorrhoeae, My- cobacterium tuberculosis, and Candida albicans) are classified as intracellular pathogens. Define this term and explain why the immune response to these pathogens differs from that to other pathogens such as Staphylococcus aureus and Strepto- coccus pneumoniae. 13. Indicate whether each of the following statements about the spleen is true or false. If you think a statement is false, ex- plain why. a. It filters antigens out of the blood. b. The marginal zone is rich in T cells, and the periarteriolar lymphoid sheath (PALS) is rich in B cells. c. It contains germinal centers. d. It functions to remove old and defective red blood cells. e. Lymphatic vessels draining the tissue spaces enter the spleen. f. Lymph node but not spleen function is affected by a knockout of the Ikaros gene 14. For each type of cell indicated (a–p), select the most appro- priate description (1–16) listed below. Each description may be used once, more than once, or not at all. Cell Types a. Common myeloid progenitor cells b. Monocytes c. Eosinophils d. Dendritic cells e. Natural killer (NK) cells f. Kupffer cells g. Lymphoid dendritic cell h. Mast cells i. Neutrophils j. M cells k. Bone-marrow stromal cells l. Lymphocytes m. NK1-T cell n. Microglial cell o. Myeloid dendritic cell p. Hematopoietic stem cell Descriptions (1) Major cell type presenting antigen to T H cells (2) Phagocytic cell of the central nervous system (3) Phagocytic cells important in the body’s defense against parasitic organisms (4) Macrophages found in the liver (5) Give rise to red blood cells (6) An antigen-presenting cell derived from monocytes that is not phagocytic (7) Generally first cells to arrive at site of inflammation (8) Secrete colony-stimulating factors (CSFs) (9) Give rise to thymocytes (10) Circulating blood cells that differentiate into macro- phages in the tissues (11) An antigen-presenting cell that arises from the same precursor as a T cell but not the same as a macrophage (12) Cells that are important in sampling antigens of the intestinal lumen (13) Nonphagocytic granulocytic cells that release various pharmacologically active substances (14) White blood cells that migrate into the tissues and play an important role in the development of allergies (15) These cells sometimes recognize their targets with the aid of an antigen-specific cell-surface receptor and sometimes by mechanisms that resemble those of nat- ural killer cells. (16) Members of this category of cells are not found in jaw- less fishes. 8536d_ch02_024-056 8/5/02 4:02 PM Page 56 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: