receptors with complement proteins controls B-cell activi-
ties gives this system a role in the highly developed acquired
immune system. Thus we have a system that straddles in-
nate and acquired immunity, contributing to each in a vari-
ety of ways.
After initial activation, the various complement compo-
nents interact, in a highly regulated cascade, to carry out a
number of basic functions (Figure 13-1) including:
a73
Lysis of cells, bacteria, and viruses
a73
Opsonization, which promotes phagocytosis of
particulate antigens
a73
Binding to specific complement receptors on cells of
the immune system, triggering specific cell functions,
inflammation, and secretion of immunoregulatory
molecules
a73
Immune clearance, which removes immune complexes
from the circulation and deposits them in the spleen and
liver
chapter 13
a73 The Functions of Complement
a73 The Complement Components
a73 Complement Activation
a73 Regulation of the Complement System
a73 Biological Consequences of Complement
Activation
a73 Complement Deficiencies
The Complement
System
T
?? ?????????? ?????? ?? ??? ????? ????????
of the humoral branch of the immune system.
Research on complement began in the 1890s,
when Jules Bordet at the Institut Pasteur in Paris showed
that sheep antiserum to the bacterium Vibrio cholerae
caused lysis of the bacteria and that heating the antiserum
destroyed its bacteriolytic activity. Surprisingly, the ability
to lyse the bacteria was restored to the heated serum by
adding fresh serum that contained no antibodies directed
against the bacterium and was unable to kill the bacterium
by itself. Bordet correctly reasoned that bacteriolytic activ-
ity requires two different substances: first, the specific an-
tibacterial antibodies, which survive the heating process,
and a second, heat-sensitive component responsible for the
lytic activity. Bordet devised a simple test for the lytic ac-
tivity, the easily detected lysis of antibody-coated red blood
cells, called hemolysis. Paul Ehrlich in Berlin indepen-
dently carried out similar experiments and coined the term
complement, defining it as “the activity of blood serum that
completes the action of antibody.” In ensuing years, re-
searchers discovered that the action of complement was
the result of interactions of a large and complex group of
proteins.
This chapter describes the complement components and
their activation pathways, the regulation of the complement
system, the effector functions of various complement com-
ponents, and the consequences of deficiencies in them. A
Clinical Focus section describes consequences of a defect in
proteins that regulate complement activity.
The Functions of Complement
Research on complement now includes more than 30 solu-
ble and cell-bound proteins. The biological activities of
this system affect both innate and acquired immunity and
reach far beyond the original observations of antibody-
mediated lysis of bacteria and red blood cells. Structural
comparisons of the proteins involved in complement path-
ways place the origin of this system in primitive organisms
possessing the most rudimentary innate immune systems.
By contrast, the realization that interaction of cellular
Poly-C9 Complex
ART TO COME
The Complement Components
The proteins and glycoproteins that compose the complement
system are synthesized mainly by liver hepatocytes, although
significant amounts are also produced by blood monocytes, tis-
sue macrophages, and epithelial cells of the gastrointestinal and
genitourinary tracts. These components constitute 5% (by
weight) of the serum globulin fraction. Most circulate in the
serum in functionally inactive forms as proenzymes, or zymo-
gens, which are inactive until proteolytic cleavage, which re-
moves an inhibitory fragment and exposes the active site. The
complement-reaction sequence starts with an enzyme cascade.
Complement components are designated by numerals
(C1–C9), by letter symbols (e.g., factor D), or by trivial
names (e.g., homologous restriction factor). Peptide frag-
ments formed by activation of a component are denoted by
small letters. In most cases, the smaller fragment resulting
from cleavage of a component is designated “a” and the larger
fragment designated “b” (e.g., C3a, C3b; note that C2 is an
exception: C2a is the larger cleavage fragment). The larger
fragments bind to the target near the site of activation, and
the smaller fragments diffuse from the site and can initiate
localized inflammatory responses by binding to specific re-
ceptors. The complement fragments interact with one an-
other to form functional complexes. Those complexes that
have enzymatic activity are designated by a bar over the num-
ber or symbol (e.g., C4b2a, C3bBb).
Complement Activation
Figure 13-2 on page 301 outlines the pathways of comple-
ment activation. The early steps, culminating in formation of
300 PART III Immune Effector Mechanisms
C5b, can occur by the classical pathway, the alternative
pathway, or the lectin pathway. The final steps that lead to a
membrane attack are the same in all pathways.
The Classical Pathway Begins with
Antigen-Antibody Binding
Complement activation by the classical pathway commonly
begins with the formation of soluble antigen-antibody com-
plexes (immune complexes) or with the binding of antibody
to antigen on a suitable target, such as a bacterial cell. IgM and
certain subclasses of IgG (human IgG1, IgG2, and IgG3) can
activate the classical complement pathway. The initial stage of
activation involves C1, C2, C3, and C4, which are present in
plasma in functionally inactive forms. Because the compo-
nents were named in order of their discovery and before their
functional roles had been determined, the numbers in their
names do not always reflect the order in which they react.
The formation of an antigen-antibody complex induces
conformational changes in the Fc portion of the IgM mole-
cule that expose a binding site for the C1 component of the
complement system. C1 in serum is a macromolecular com-
plex consisting of C1q and two molecules each of C1r and
C1s, held together in a complex (C1qr
2
s
2
) stabilized by Ca
2H11001
ions. The C1q molecule is composed of 18 polypeptide
chains that associate to form six collagen-like triple helical
arms, the tips of which bind to exposed C1q-binding sites in
the C
H
2 domain of the antibody molecule (Figure 13-3, on
page 302). Each C1r and C1s monomer contains a catalytic
domain and an interaction domain; the latter facilitates in-
teraction with C1q or with each other.
Each C1 molecule must bind by its C1q globular heads to
at least two Fc sites for a stable C1-antibody interaction to
FIGURE 13-1 The multiple activities of the complement system.
Serum complement proteins and membrane-bound complement
receptors partake in a number of immune activities: lysis of foreign
cells by antibody-dependent or antibody-independent pathways;
opsonization or uptake of particulate antigens, including bacteria, by
phagocytes; activation of inflammatory responses; and clearance of
circulating immune complexes by cells in the liver and spleen.
Soluble complement proteins are schematically indicated by a trian-
gle and receptors by a semi-circle; no attempt is made to differenti-
ate among individual components of the complement system here.
Complement
receptor
Blood
Tissue
Phagocyte Phagocyte
Degranulation
Target cell
Ag-Ab
complex
Complement
Extravasation
Bacteria
LYSIS OPSONIZATION ACTIVATION OF INFLAMMATORY
RESPONSE
CLEARANCE OF
IMMUNE COMPLEXES
occur. When pentameric IgM is bound to antigen on a target
surface it assumes the so-called “staple” configuration, in
which at least three binding sites for C1q are exposed. Circu-
lating IgM, however, exists as a planar configuration in which
the C1q-binding sites are not exposed (Figure 13-4, on page
302) and therefore cannot activate the complement cascade.
An IgG molecule, on the other hand, contains only a single
C1q-binding site in the C
H
2 domain of the Fc, so that firm C1q
binding is achieved only when two IgG molecules are within
30–40 nm of each other on a target surface or in a complex,
The Complement System CHAPTER 13 301
providing two attachment sites for C1q. This difference ac-
counts for the observation that a single molecule of IgM
bound to a red blood cell can activate the classical complement
pathway and lyse the red blood cell while some 1000 mole-
cules of IgG are required to assure that two IgG molecules are
close enough to each other on the cell surface to initiate C1q
binding.
The intermediates in the classical activation pathway are
depicted schematically in Figure 13-5 (page 303). Binding of
C1q to Fc binding sites induces a conformational change in
FIGURE 13-2 Overview of the complement activation pathways.
The classical pathway is initiated when C1 binds to antigen-antibody
complexes. The alternative pathway is initiated by binding of spon-
taneously generated C3b to activating surfaces such as microbial cell
walls. The lectin pathway is initiated by binding of the serum protein
MBL to the surface of a pathogen. All three pathways generate C3
and C5 convertases and bound C5b, which is converted into a mem-
brane-attack complex (MAC) by a common sequence of terminal
reactions. Hydrolysis of C3 is the major amplification step in all path-
ways, generating large amounts of C3b, which forms part of C5 con-
vertase. C3b also can diffuse away from the activating surface and
bind to immune complexes or foreign cell surfaces, where it func-
tions as an opsonin.
+
C4
C2
C3b
C3
C5 C5b
C9
C8
C7
C6
C3
C3b
C3bB
Factor B
Factor D
C4b2a
C3bBb C3bBb3b
C4b2a3b
C5 convertase
C5 convertase
C3 convertase
C3 convertase
Activated
C1
MBL-associated proteases
(MASP1 + 2) bind MBL,
generate activated
C1-like complex
Mannose-binding
lectin (MBL)
binds foreign surface
C1 binds
antigen-antibody
complex
Classical
pathway
Alternative
pathway
Lectin
pathway
Major
amplification
step
Membrane
attack
complex
Spontaneous, slow,
small amounts
(text continues on page 304)
302 PART III Immune Effector Mechanisms
(b)
FIGURE 13-3 Structure of the C1 macromolecular complex. (a) Di-
agram of C1qr
2
s
2
complex. A C1q molecule consists of 18 polypep-
tide chains arranged into six triplets, each of which contains one A,
one B, and one C chain. Each C1r and C1s monomer contains a cat-
alytic domain with enzymatic activity and an interaction domain that
facilitates binding with C1q or with each other. (b) Electron micro-
graph of C1q molecule showing stalk and six globular heads. [Part (b)
from H. R. Knobel et al., 1975, Eur. J. Immunol. 5:78.]
(a)
C1r
C1s
Stalk
Heads
C1q
FIGURE 13-4 Models of pentameric IgM in planar form (a) and
“staple” form (b). Several C1q-binding sites in the Fc region are
accessible in the staple form, whereas none are exposed in the pla-
nar form. Electron micrographs of IgM antiflagellum antibody
bound to flagella, showing the planar form (c) and staple form (d).
[From A. Feinstein et al., 1981, Monogr. Allergy, 17:28, and 1981,
Ann. N.Y. Acad. Sci. 190:1104.]
(a) (b)
(c) (d)
The Complement System CHAPTER 13 303
VISUALIZING CONCEPTS
FIGURE 13-5 Schematic diagram of intermediates in the classi-
cal pathway of complement activation. The completed membrane-
attack complex (MAC, bottom right) forms a large pore in the
membrane.
Poly-C9
3
5
C5b binds C6, initiating the formation of the membrane-attack
complex
C1s cleaves C4 and C2. Cleaving C4 exposes the binding site
for C2. C4 binds the surface near C1 and C2 binds C4,
forming C3 convertase
C3 convertase hydrolyzes many C3 molecules. Some combine
with C3 convertase to form C5 convertase
4
The C3b component of C5 convertase binds C5, permitting
C4b2a to cleave C5
1
C1q binds antigen-bound antibody. C1r activates auto-
catalytically and activates the second C1r; both activate C1s
+
C3 convertase
C5 convertase
C5 convertase
+
C5
C5b C5a
C1q
F
C
Antibody
C1r
2
s
2
C4b2a
C4b2a C4b2a3b
C1qr
2
s
2
C4a
C2
C2b
C4
C3
C3b C3a
2
C9
C5b678
C8
C5b678C5b C567
C7C6
Membrane attack complex
C1r that converts C1r to an active serine protease enzyme,
C1rH33526, which then cleaves C1s to a similar active enzyme, C11sH33526.
C11sH33526 has two substrates, C4 and C2. The C4 component is a
glycoprotein containing three polypeptide chains H9251,H9252, and H9253.
C4 is activated when C11sH33526 hydrolyzes a small fragment (C4a)
from the amino terminus of the H9251 chain, exposing a binding
site on the larger fragment (C4b). The C4b fragment attaches
to the target surface in the vicinity of C1, and the C2 proen-
zyme then attaches to the exposed binding site on C4b, where
the C2 is then cleaved by the neighboring C11sH33526; the smaller
fragment (C2b) diffuses away. The resulting C4b2aH33526H33526 complex
is called C3 convertase, referring to its role in converting the
C3 into an active form. The smaller fragment from C4 cleav-
age, C4a, is an anaphylatoxin, or mediator of inflammation,
which does not participate directly in the complement cas-
cade; the anaphylatoxins, which include the smaller frag-
ments of C4, C3, and C5 are described below.
The native C3 component consists of two polypeptide
chains, H9251 and H9252. Hydrolysis of a short fragment (C3a) from
the amino terminus of the H9251 chain by the C3 convertase gen-
erates C3b (Figure 13-6). A single C3 convertase molecule can
generate over 200 molecules of C3b, resulting in tremendous
amplification at this step of the sequence. Some of the C3b
binds to C4b2aH33526H33526 to form a trimolecular complex C4bH335262aH335263bH33526,
called C5 convertase. The C3b component of this complex
binds C5 and alters its conformation, so that the C4b2aH33526H33526 com-
ponent can cleave C5 into C5a, which diffuses away, and C5b,
which attaches to C6 and initiates formation of the membrane-
attack complex in a sequence described later. Some of the
C3b generated by C3 convertase activity does not associate
with C4b2aH33526H33526; instead it diffuses away and then coats immune
complexes and particulate antigens, functioning as an opsonin
as described in the Clinical Focus. C3b may also bind directly
to cell membranes.
The Alternative Pathway Is
Antibody-Independent
The alternative pathway generates bound C5b, the same
product that the classical pathway generates, but it does so
without the need for antigen-antibody complexes for initia-
tion. Because no antibody is required, the alternative path-
way is a component of the innate immune system. This
major pathway of complement activation involves four
serum proteins: C3, factor B, factor D, and properdin. The al-
ternative pathway is initiated in most cases by cell-surface
constituents that are foreign to the host (Table 13-1). For ex-
ample, both gram-negative and gram-positive bacteria have
cell-wall constituents that can activate the alternative path-
way. The intermediates in the alternative pathway for gener-
ating C5b are shown schematically in Figure 13-7 (page 306).
In the classical pathway, C3 is rapidly cleaved to C3a and
C3b by the enzymatic activity of the C3 convertase. In the al-
ternative pathway, serum C3, which contains an unstable
thioester bond, is subject to slow spontaneous hydrolysis to
yield C3a and C3b. The C3b component can bind to foreign
surface antigens (such as those on bacterial cells or viral par-
ticles) or even to the host’s own cells (see Figure 13-6c). The
membranes of most mammalian cells have high levels of
sialic acid, which contributes to the rapid inactivation of
bound C3b molecules on host cells; consequently this bind-
ing rarely leads to further reactions on the host cell mem-
brane. Because many foreign antigenic surfaces (e.g., bac-
terial cell walls, yeast cell walls, and certain viral envelopes)
have only low levels of sialic acid, C3b bound to these sur-
faces remains active for a longer time. The C3b present on the
surface of the foreign cells can bind another serum protein
called factor B to form a complex stabilized by Mg
2H11001
.Bind-
ing to C3b exposes a site on factor B that serves as the sub-
304 PART III Immune Effector Mechanisms
FIGURE 13-6 Hydrolysis of C3 by C3 convertase C4b2a (a) Native
C3. (b) Activated C3 showing site of cleavage by C4b2a resulting in
production of the C3a and C3b fragments. (c) A labile internal
thioester bond in C3 is activated as C3b is formed, allowing the C3b
fragment to bind to free hydroxyl or amino groups (R) on a cell mem-
brane. Bound C3b exhibits various biological activities, including
binding of C5 and binding to C3b receptors on phagocytic cells.
(b)(a) (c)
C
O
S
S
S
S
S
S
S
S
S
S
S
S
S
α
β
C3
C4b2a
C3a
C
+
O
–
S
Activated C3b
C
O
SH
R
O
Bound C3b
Cell membrane
strate for an enzymatically active serum protein called factor
D. Factor D cleaves the C3b-bound factor B, releasing a small
fragment (Ba) that diffuses away and generating C33bBbH33526H33526.The
C33bBbH33526H33526 complex has C3 convertase activity and thus is analo-
gous to the C4b2aH33526H33526 complex in the classical pathway. The C3
convertase activity of C33bBbH33526H33526 has a half-life of only 5 minutes
unless the serum protein properdin binds to it, stabilizing
it and extending the half-life of this convertase activity to
30 minutes.
The C33bBbH33526H33526 generated in the alternative pathway can acti-
vate unhydrolyzed C3 to generate more C3b autocatalytically.
As a result, the initial steps are repeated and amplified, so
that more than 2 H11003 10
6
molecules of C3b can be deposited
on an antigenic surface in less than 5 minutes. The C3 con-
vertase activity of C33bBbH33526H33526 generates the C33b
H33526
Bb3b
H33526H33526H33526
complex,
which exhibits C5 convertase activity, analogous to the
C4b2a
H33526H33526
3b
H33526
complex in the classical pathway. The nonenzy-
matic C3b component binds C5, and the Bb component
subsequently hydrolyzes the bound C5 to generate C5a and
C5b (see Figure 13-7); the latter binds to the antigenic surface.
The Lectin Pathway Originates With Host
Proteins Binding Microbial Surfaces
Lectins are proteins that recognize and bind to specific car-
bohydrate targets. (Because the lectin that activates comple-
ment binds to mannose residues, some authors designate this
the MBLectin pathway or mannan-binding lectin pathway.)
The lectin pathway, like the alternative pathway, does not de-
pend on antibody for its activation. However, the mechanism
is more like that of the classical pathway, because after initia-
tion, it proceeds, through the action of C4 and C2, to pro-
duce a C5 convertase (see Figure 13-2).
The lectin pathway is activated by the binding of man-
nose-binding lectin (MBL) to mannose residues on glyco-
proteins or carbohydrates on the surface of microorganisms
including certain Salmonella, Listeria, and Neisseria strains,
as well as Cryptococcus neoformans and Candida albicans.
MBL is an acute phase protein produced in inflammatory
responses. Its function in the complement pathway is similar
to that of C1q, which it resembles in structure. After MBL
binds to the surface of a cell or pathogen, MBL-associated
serine proteases, MASP-1 and MASP-2, bind to MBL. The ac-
tive complex formed by this association causes cleavage and
activation of C4 and C2. The MASP-1 and -2 proteins have
structural similarity to C1r and C1s and mimic their activi-
ties. This means of activating the C2–C4 components to
form a C5 convertase without need for specific antibody
binding represents an important innate defense mechanism
comparable to the alternative pathway, but utilizing the ele-
ments of the classical pathway except for the C1 proteins.
The Three Complement Pathways Converge
at the Membrane-Attack Complex
The terminal sequence of complement activation involves
C5b, C6, C7, C8, and C9, which interact sequentially to form
a macromolecular structure called the membrane-attack
complex (MAC). This complex forms a large channel
through the membrane of the target cell, enabling ions and
small molecules to diffuse freely across the membrane.
The end result of activating the classical, alternative, or
lectin pathways is production of an active C5 convertase.
This enzyme cleaves C5, which contains two protein chains,
H9251 and H9252. After binding of C5 to the nonenzymatic C3b com-
ponent of the convertase, the amino terminus of the H9251 chain
is cleaved. This generates the small C5a fragment, which dif-
fuses away, and the large C5b fragment, which binds to the
surface of the target cell and provides a binding site for the
subsequent components of the membrane-attack complex
(see Figure 13-5, step 5). The C5b component is extremely la-
bile and becomes inactive within 2 minutes unless C6 binds
to it and stabilizes its activity.
Up to this point, all the complement reactions take place
on the hydrophilic surface of membranes or on immune
complexes in the fluid phase. As C5b6 binds to C7, the result-
ing complex undergoes a hydrophilic-amphiphilic structural
transition that exposes hydrophobic regions, which serve as
binding sites for membrane phospholipids. If the reaction
occurs on a target-cell membrane, the hydrophobic binding
sites enable the C5b67 complex to insert into the phospho-
lipid bilayer. If, however, the reaction occurs on an immune
The Complement System CHAPTER 13 305
TABLE 13-1
Initiators of the alternative pathway
of complement activation
PATHOGENS AND PARTICLES OF MICROBIAL ORIGIN
Many strains of gram-negative bacteria
Lipopolysaccharides from gram-negative bacteria
Many strains of gram-positive bacteria
Teichoic acid from gram-positive cell walls
Fungal and yeast cell walls (zymosan)
Some viruses and virus-infected cells
Some tumor cells (Raji)
Parasites (trypanosomes)
NONPATHOGENS
Human IgG, IgA, and IgE in complexes
Rabbit and guinea pig IgG in complexes
Cobra venom factor
Heterologous erythrocytes (rabbit, mouse, chicken)
Anionic polymers (dextran sulfate)
Pure carbohydrates (agarose, inulin)
SOURCE: Adapted from M. K. Pangburn, 1986, in Immunobiology of the
Complement System, Academic Press.
complex or other noncellular activating surface, then the hy-
drophobic binding sites cannot anchor the complex and it is
released. Released C5b67 complexes can insert into the mem-
brane of nearby cells and mediate “innocent-bystander” lysis.
Regulator proteins normally prevent this from occurring, but
in certain diseases cell and tissue damage may result from in-
nocent-bystander lysis. A hemolytic disorder resulting from
deficiency in a regulatory protein is explained in the Clinical
Focus section and an autoimmune process in which immune
306 PART III Immune Effector Mechanisms
complexes mediate tissue damage will be considered in
Chapter 20.
Binding of C8 to membrane-bound C5b67 induces a con-
formational change in C8, so that it too undergoes a hy-
drophilic-amphiphilic structural transition, exposing a
hydrophobic region, which interacts with the plasma mem-
brane. The C5b678 complex creates a small pore, 10 ? in di-
ameter; formation of this pore can lead to lysis of red blood
cells but not of nucleated cells. The final step in formation of
VISUALIZING CONCEPTS
3
2
4
1
Factor B binds C3a, exposes site acted
on by Factor D. Cleavage generates
C3bBb, which has C3 convertase
activity
Binding of properdin stabilizes
convertase
Convertase generates C3b; some binds
to C3 convertase activating C5'
convertase. C5b binds to antigenic
surface
C3 hydrolyzes spontaneously, C3b
fragment attaches to foreign surface
+ Properdin
+
C3
+
C3
C3b C3a
+
C5 C5b
Membrane
attack
complex
C5a
Factor B
Factor D
C3 convertase
C3bBb
C3bBb3B
C3 convertase
FIGURE 13-7 Schematic diagram of intermediates in the for-
mation of bound C5b by the alternative pathway of complement
activation. The C3bBb complex is stabilized by binding of prop-
erdin. Conversion of bound C5b to the membrane-attack complex
occurs by the same sequence of reactions as in the classical path-
way (see Figure 13-5).
the MAC is the binding and polymerization of C9, a per-
forin-like molecule, to the C5b678 complex. As many as
10–17 molecules of C9 can be bound and polymerized by
a single C5b678 complex. During polymerization, the C9
molecules undergo a hydrophilic-amphiphilic transition, so
that they too can insert into the membrane. The completed
MAC, which has a tubular form and functional pore size of
70–100 ?, consists of a C5b678 complex surrounded by a
poly-C9 complex (Figure 13-8). Since ions and small mole-
cules can diffuse freely through the central channel of the
MAC, the cell cannot maintain its osmotic stability and is
killed by an influx of water and loss of electrolytes.
Regulation of the Complement
System
Because many elements of the complement system are capa-
ble of attacking host cells as well as foreign cells and microor-
ganisms, elaborate regulatory mechanisms have evolved to
restrict complement activity to designated targets. A general
mechanism of regulation in all complement pathways is the
inclusion of highly labile components that undergo sponta-
neous inactivation if they are not stabilized by reaction with
other components. In addition, a series of regulatory pro-
teins can inactivate various complement components (Table
13-2). For example, the glycoprotein C1 inhibitor (C1Inh)
can form a complex with C1r
2
s
2
, causing it to dissociate from
C1q and preventing further activation of C4 or C2 (Figure
13-9a(1)).
The reaction catalyzed by the C3 convertase enzymes of the
classical, lectin, and alternative pathways is the major amplifi-
cation step in complement activation, generating hundreds of
molecules of C3b. The C3b generated by these enzymes has the
potential to bind to nearby cells, mediating damage to the
healthy cells by causing their opsonization by phagocytic cells
bearing C3b receptors or by induction of the membrane-
attack complex. Damage to normal host cells is prevented be-
cause C3b undergoes spontaneous hydrolysis by the time it has
diffused 40 nm away from the C4b2aH33526H33526 or C33bBbH33526H33526 convertase en-
zymes, so that it can no longer bind to its target site. The po-
tential destruction of healthy host cells by C3b is further
limited by a family of related proteins that regulate C3 conver-
tase activity in the classical and alternative pathways. These
regulatory proteins all contain repeating amino acid sequences
(or motifs) of about 60 residues, termed short consensus repeats
(SCRs). All these proteins are encoded at a single location on
chromosome 1 in humans, known as the regulators of comple-
ment activation (RCA) gene cluster.
In the classical and lectin pathways, three structurally dis-
tinct RCA proteins act similarly to prevent assembly of C3
convertase (Figure 13-9a(2)). These regulatory proteins in-
clude soluble C4b-binding protein (C4bBP) and two mem-
brane-bound proteins, complement receptor type 1 (CR1)
and membrane cofactor protein (MCP). Each of these regu-
latory proteins binds to C4b and prevents its association with
C2a. Once C4bBP, CR1, or MCP is bound to C4b, another
regulatory protein, factor I, cleaves the C4b into bound C4d
and soluble C4c (Figure 13-9a(3)). A similar regulatory se-
quence operates to prevent assembly of the C3 convertase
C33bBbH33526H33526 in the alternative pathway. In this case CR1, MCP, or a
regulatory component called factor H binds to C3b and pre-
vents its association with factor B (Figure 13-9a(4)). Once
CR1, MCP, or factor H is bound to C3b, factor I cleaves the
C3b into a bound iC3b fragment and a soluble C3f fragment.
Further cleavage of iC3b by factor I releases C3c and leaves
C3dg bound to the membrane (Figure 13-9a(5)). The mole-
cular events involved in regulation of cell-bound C4b and
C3b are depicted in Figure 13-10 (page 310).
The Complement System CHAPTER 13 307
(a)
FIGURE 13-8 (a) Photomicrograph of poly-C9 complex formed by
in vitro polymerization of C9. (b) Photomicrograph of complement-
induced lesions on the membrane of a red blood cell. These lesions
result from formation of membrane-attack complexes. [Part (a) from
E. R. Podack, 1986, in Immunobiology of the Complement System,
Academic Press; part (b) from J. Humphrey and R. Dourmashkin,
1969, Adv. Immunol. 11:75.]
(b)
Several RCA proteins also act on the assembled C3 con-
vertase, causing it to dissociate; these include the previously
mentioned C4bBP, CR1, and factor H. In addition, decay-
accelerating factor (DAF or CD55), which is a glycoprotein an-
chored covalently to a glycophospholipid membrane protein,
has the ability to dissociate C3 convertase. The consequences
of DAF deficiency are described in the Clinical Focus section.
Each of these RCA proteins accelerates decay (dissociation) of
C3 convertase by releasing the component with enzymatic ac-
tivity (C2a or Bb) from the cell-bound component (C4b or
C3b). Once dissociation of the C3 convertase occurs, factor I
cleaves the remaining membrane-bound C4b or C3b compo-
nent, irreversibly inactivating the convertase (Figure 13-9b).
Regulatory proteins also operate at the level of the mem-
brane-attack complex. The potential release of the C5b67
complex poses a threat of innocent-bystander lysis to healthy
cells. A number of serum proteins counter this threat by
binding to released C5b67 and preventing its insertion into
the membrane of nearby cells. A serum protein called S pro-
tein can bind to C5b67, inducing a hydrophilic transition
and thereby preventing insertion of C5b67 into the mem-
brane of nearby cells (Figure 13-9c(1)).
Complement-mediated lysis of cells is more effective if
the complement is from a species different from that of the
cells being lysed. This phenomenon depends on two mem-
brane proteins that block MAC formation. These two pro-
teins, present on the membrane of many cell types, are
homologous restriction factor (HRF) and membrane inhibitor
of reactive lysis (MIRL or CD59). Both HRF and MIRL pro-
tect cells from nonspecific complement-mediated lysis by
binding to C8, preventing assembly of poly-C9 and its inser-
tion into the plasma membrane (Figure 13-9c(2)). However,
this inhibition occurs only if the complement components
are from the same species as the target cells. For this reason,
MIRL and HRF are said to display homologous restriction,
for which the latter was named. As discussed in Chapter 21,
homologous restriction poses a barrier to the use of organs
from other species for clinical transplantation.
308 PART III Immune Effector Mechanisms
TABLE 13-2 Proteins that regulate the complement system
Type of Pathway
Protein protein affected Immunologic function
C1 inhibitor (C1Inh) Soluble Classical Serine protease inhibitor: causes C1r
2
s
2
to dissociate from C1q
C4b-binding protein Soluble Classical and lectin Blocks formation of C3 convertase by
(C4bBP)* binding C4b; cofactor for cleavage of
C4b by factor I
Factor H* Soluble Alternative Blocks formation of C3 convertase by
binding C3b; cofactor for cleavage of
C3b by factor I
Complement-receptor Block formation of C3 convertase by
type 1 (CR1)* Membrane Classical, alternative, binding C4b or C3b; cofactor for
Membrane-cofactor bound and lectin factor I-catalyzed cleavage of C4b
protein (MCP)* or C3b C3bBb
Decay-accelerating Membrane Classical, alternative, Accelerates dissociation of C4b2a and
factor (DAE or CD55)* bound and lectin C3bBb (classical and alternative C3
convertases)
Factor-I Soluble Classical, alternative, Serine protease: cleaves C4b or C3b
and lectin using C4bBP, CR1, factor H, DAE,
or MCP as cofactor
S protein Soluble Terminal Binds soluble C5b67 and prevents its
insertion into cell membrane
Homologous restriction
factor (HRF) Membrane Terminal Bind to C5b678 on autologous cells,
Membrane inhibitor of bound blocking binding of C9
reactive lysis (MIRL
or CD59)*
Anaphylatoxin inactivator Soluble Effector Inactivates anaphylatoxin activity of C3a,
C4a, and C5a by carboxypeptidase N
removal of C-terminal Arg
*An RCA (regulator of complement activation) protein. In humans, all RCA proteins are encoded on chromosome 1 and contain short consensus repeats.
}
}
The Complement System CHAPTER 13 309
VISUALIZING CONCEPTS
FIGURE 13-9 Regulation of the complement system by regulatory proteins (black).
Poly-C9
Association of C4b and C2a is blocked by binding C4b-binding
protein (C4bBP), complement receptor type I, or membrane
cofactor protein (MCP)
4
In alternative pathway, CR1, MCP, or Factor H prevent
binding of C3b and Factor B
1
C1 inhibitor (C1Iab) binds C1r
2
s
2
, causing dissociation
from C1q
C3 convertases are dissociated by C4bBP, CR1, Factor H, and
decay-accelerating Factor (DAF)
+
C3 convertase
C2a C4b
C4c
C4d
2
Inhibitor-bound C4b is cleaved by Factor 1
3
Inhibitor-bound C3b is cleaved by Factor 1
5
C9
C5b678
C8
HRF,
MIRL
C5b678C5b67
S protein
C5b67
Membrane attack complex
C1Inh
C1r
2
s
2
Antibody
C1qr
2
s
2
(a) Before assembly of convertase activity
(b) After assembly of convertase
1
S protein prevents insertion of C5b67 MAC component into
the membrane
2
Homologous restriction factor
(HRF) and membrane inhibitor
of reactive lysis (MIRL or
CD59) bind C8
1
, preventing
assembly of poly-C9 and
blocking formation of MAC
(c) Regulation at assembly of membrane-attack complex (MAC)
Regulation of the Complement System
Factor 1
C4bBP, CRI, or MCP
+
C3 convertase
Factor
B
C3b
C3c
Dissociation of convertase;
remaining C4b or C3b
cleaved by Factor 1
C3f
iC3b
C3dg
Factor 1
C4bBP, CR2
Factor H, DAF
Factor 1
CRI, MCP, Factor H
Cannot attack nearby cells
Biological Consequences of
Complement Activation
Complement serves as an important mediator of the hu-
moral response by amplifying the response and converting
it into an effective defense mechanism to destroy invad-
ing microorganisms. The MAC mediates cell lysis, while
other complement components or split products participate
in the inflammatory response, opsonization of antigen,
viral neutralization, and clearance of immune complexes
(Table 13-3, page 312).
Many of the biological activities of the complement sys-
tem depend on the binding of complement fragments to
complement receptors, which are expressed by various cells.
In addition, some complement receptors play an important
role in regulating complement activity by binding biologi-
cally active complement components and degrading them
into inactive products. The complement receptors and their
primary ligands, which include various complement compo-
nents and their proteolytic breakdown products, are listed in
Table 13-4 (page 312).
The Membrane-Attack Complex Can Lyse a
Broad Spectrum of Cells
The membrane-attack complex formed by complement acti-
vation can lyse gram-negative bacteria, parasites, viruses,
erythrocytes, and nucleated cells. Because the alternative and
lectin pathways of activation generally occur without an ini-
tial antigen-antibody interaction, these pathways serve as im-
portant innate immune defenses against infectious micro-
organisms. The requirement for an initial antigen-antibody
reaction in the classical pathway supplements these nonspe-
cific innate defenses with a more specific defense mecha-
nism. In some instances, the requirement for antibody in the
activating event may be supplied by so-called natural anti-
bodies, which are raised against common components of
ubiquitous microbes.
The importance of cell-mediated immunity in host de-
fense against viral infections has been emphasized in previ-
ous chapters. Nevertheless, antibody and complement do
play a role in host defense against viruses and are often
crucial in containing viral spread during acute infection
and in protecting against reinfection. Most—perhaps
310 PART III Immune Effector Mechanisms
FIGURE 13-10 Inactivation of bound C4b and C3b by regulatory
proteins of the complement system. (a) In the classical pathway,
C4bBP (C4b-binding protein), CR1 (complement receptor type 1),
or MCP (membrane cofactor protein) bind to C4b and act as cofac-
tors for factor I–mediated cleavage of C4b. (b) In the alternative
pathway, factor H, CR1, or MCP bind to Ccb and act as cofactors
for factor I–mediated cleavage of C4b. Free diffusible fragments
are shown in dark shades; membrane bound components in light
shades.
(a)
Factor I
Cell membrane
Cell membrane
C
NH
O
S
S
S
S
S
S
S
S
C4d
C4c
S
S
S
S
S
S
S
S
C4c
S
S
S
S
S
S
S
S
Bound C4b
SH
C4bBP or
CR1 or MCP
S
S
S
S
Bound C3b
(b)
Factor I
Factor H or
CR1 or MCP
C
O
SH
R
O
S
S
S
S
iC3b
C3f
Factor I
S
S
S
S
C3c
S
S
S
S
C3c
C
NH
O
C
O
SH
R
O C
O
SH
R
O
C3dg
all—enveloped viruses are susceptible to complement-
mediated lysis. The viral envelope is largely derived from
the plasma membrane of infected host cells and is there-
fore susceptible to pore formation by the membrane-
attack complex. Among the pathogenic viruses susceptible
to lysis by complement-mediated lysis are herpesviruses,
orthomyxoviruses, paramyxoviruses, and retroviruses.
The complement system is generally quite effective in
lysing gram-negative bacteria (Figure 13-11). However,
some gram-negative bacteria and most gram-positive bac-
teria have mechanisms for evading complement-mediated
damage (Table 13-5). For example, a few gram-negative bac-
teria can develop resistance to complement-mediated lysis
that correlates with the virulence of the organism. In Es-
cherichia coli and Salmonella, resistance to complement is as-
sociated with the smooth bacterial phenotype, which is
characterized by the presence of long polysaccharide side
chains in the cell-wall lipopolysaccharide (LPS) component.
It has been proposed that the increased LPS in the wall of re-
sistant strains may prevent insertion of the MAC into the
bacterial membrane, so that the complex is released from the
bacterial cell rather than forming a pore. Strains of Neisseria
The Complement System CHAPTER 13 311
plement-mediated cell lysis, but act at
different stages of the process. DAF in-
hibits cell lysis by causing dissociation
and inactivation of the C3 convertases of
the classical, lectin, and alternative path-
ways (see Figure 13-9b). MIRL acts later
in the pathway by binding to the C5b678
complex, which inhibits C9 binding and
prevents formation of the pores that de-
stroy the cell under attack. Both proteins
are expressed on erythrocytes as well as
a number of other hematopoetic cell
types. Deficiency in these proteins leads
to highly increased sensitivity of host
cells to the lytic effects of the host’s com-
plement activity. PNH, the clinical con-
sequence of deficiency in DAF and MIRL,
is a chronic disease with a mean sur-
vival time between 10 and 15 years. The
most common causes of mortality in
PNH are venous thrombosis affecting
hepatic veins and progressive bone-
marrow failure.
An obvious question about this rare
but serious disease concerns the fact
that two different proteins are involved
in its pathogenesis. The simultaneous
occurrence of a genetic defect in each of
them would be rarer than the 1 in
100,000 incidence of PNH. The answer
is that neither protein itself is defective in
PNH; the defect lies in a posttransla-
tional modification of the peptide anchor
that binds them to the cell membrane.
While most proteins that are expressed
on the surface of cells have hydrophobic
sequences that traverse the lipid bilayer
in the cell membrane, some proteins are
bound by glycolipid anchors (glycosyl
phosphatidylinositol, or GPI) attached to
amino acid residues in the protein. With-
out the ability to form GPI anchors, pro-
teins that attach in this manner will be
absent from the cell surface, including
both DAF and MIRL.
The defect identified in PNH lies early
in the enzymatic path to formation of a
GPI anchor and resides in the pig-a gene
(phosphatidylinositol glycan comple-
mentation class A gene). Transfection of
cells from PNH patients with an intact
pig-a gene restored the cells’ resistance
to host complement lysis. Examination
of pig-a sequences in PNH patients re-
veals a number of different defects in
this X-linked gene, indicating somatic
rather than genetic origin of the defect.
This description of PNH underscores
the fact that the complement system is a
powerful defender of the host but also a
dangerous one. Complex systems of reg-
ulation are necessary to protect host
cells from the activated complement
complexes generated to lyse intruders.
Common conditions
associated with deficiency in the comple-
ment components include increased
susceptibility to bacterial infections and
systemic lupus erythematosus which is
related to the inability to clear immune
complexes. Deficiency in the proteins
that regulate complement activity can
cause equally serious disorders. An ex-
ample is paroxymal nocturnal hemoglo-
binuria, or PNH, which manifests as
increased fragility of erythrocytes, lead-
ing to chronic hemolytic anemia, pancy-
topenia (loss of blood cells of all types)
and venous thrombosis (formation of
blood clots). The name PNH derives
from the presence of hemoglobin in the
urine, most commonly observed in the
first urine passed after a night’s sleep.
The cause of PNH is a general defect in
synthesis of cell-surface proteins, which
affects the expression of two regulators
of complement, DAF (decay accelerating
factor or CD55) and MIRL (membrane
inhibitor of reactive lysis or CD59).
DAF and MIRL are cell-surface pro-
teins that function as inhibitors of com-
CLINICAL FOCUS
Paroxymal Nocturnal
Hemoglobinuria: a Defect in
Regulation of Complement
Lysis
312 PART III Immune Effector Mechanisms
TABLE 13-3 Summary of biological effects mediated by complement products
Effect Complement product mediating*
Cell lysis C5b–9, the membrane-attack complex (MAC)
Inflammatory response
Degranulation of mast cells and basophils
?
C3a,C4a, and C5a (anaphylatoxins)
Degranulation of eosinophils C3a, C5a
Extravasation and chemotaxis of leukocytes at inflammatory site C3a, C5a, C5b67
Aggregation of platelets C3a, C5a
Inhibition of monocyte/macrophage migration and induction Bb
of their spreading
Release of neutrophils from bone marrow C3c
Release of hydrolytic enzymes from neutrophils C5a
Increased expression of complement receptors C5a
type 1 and 3 (CR1 and CR3) on neutrophils
Opsonization of particulate antigens, increasing their phagocytosis C3b, C4b, iC3b
Viral neutralization C3b, C5b–9 (MAC)
Solubilization and clearance of immune complexes C3b
*Boldfaced component is most important in mediating indicated effect.
?
Degranulation leads to release of histamine and other mediators that induce contraction of smooth muscle and increased permeability of vessels.
TABLE 13-4 Complement-binding receptors
Receptor Major ligands Activity Cellular distribution
CR1 (CD35)C3b, C4b Blocks formation of C3 Erythrocytes, neutrophils,
convertase; binds immune monocytes, macrophages,
complexes to cells eosinophils, follicular dendritic
cells, B cells, some T cells
CR2 (CD21)C3d, C3dg,* Part of B-cell coreceptor; B cells, follicular dendritic
iC3b binds Epstein-Barr virus cells, some T cells
CR3 (CD11b/18) Bind cell-adhesion molecules Monocytes, macrophages,
iC3b on neutrophils, facilitating their neutrophils, natural killer
CR4 (CD11c/18) extravasation; bind immune cells, some T cells
complexes, enhancing their
phagocytosis
C3a/C4a receptor C3a, C4a Induces degranulation of mast Mast cells, basophils, granulocytes
cells and basophils
C5a receptor C5a Induces degranulation of mast Mast cells, basophils, granulocytes,
cells and basophils monocytes, macrophages,
platelets, endothelial cells
*Cleavage of C3dg by serum proteases generates C3d and C3g.
}
gonorrheae resistant to complement-mediated killing have
been associated with disseminated gonococcal infections in
humans. Some evidence suggests that the membrane pro-
teins of resistant Neisseria strains undergo noncovalent in-
teractions with the MAC that prevent its insertion into the
outer membrane of the bacterial cells. These examples of
resistant gram-negative bacteria are the exception; most
gram-negative bacteria are susceptible to complement-
mediated lysis.
Gram-positive bacteria are generally resistant to comple-
ment-mediated lysis because the thick peptidoglycan layer in
their cell wall prevents insertion of the MAC into the inner
membrane. Although complement activation can occur on
the cell membrane of encapsulated bacteria such as Strepto-
coccus pneumoniae, the capsule prevents interaction between
C3b deposited on the membrane and the CR1 on phagocytic
cells. Some bacteria possess an elastase that inactivates C3a
and C5a, preventing these split products from inducing an
inflammatory response. In addition to these mechanisms of
evasion, various bacteria, viruses, fungi, and protozoans con-
tain proteins that can interrupt the complement cascade on
their surfaces, thus mimicking the effects of the normal com-
plement regulatory proteins C4bBP, CR1, and DAF.
Lysis of nucleated cells requires formation of multiple
membrane attack complexes, whereas a single MAC can lyse
a red blood cell. Many nucleated cells, including the majority
of cancer cells, can endocytose the MAC. If the complex is
removed soon enough, the cell can repair any membrane
The Complement System CHAPTER 13 313
(a)
Alive
FIGURE 13-11 Scanning electron micrographs of E. coli showing
(a) intact cells and (b, c) cells killed by complement-mediated lysis.
Note membrane blebbing on lysed cells. [From R. D. Schreiber et
al., 1979, J. Exp. Med. 149:870.]
(b)
Killed
(c)
Killed
TABLE 13-5 Microbial evasion of complement-mediated damage
Microbial component Mechanism of evasion Examples
GRAM-NEGATIVE BACTERIA
Long polysaccharide chains Side chains prevent insertion of Resistant strains of E. coli and
in cell-wall LPS MAC into bacterial membrane Salmonella
Outer membrane protein MAC interacts with membrane Resistant strains of Neisseria
protein and fails to insert into gonorrhoeae
bacterial membrane
Elastase Anaphylatoxins C3a and C5a are Pseudomonas aeruginosa
inactivated by microbial elastase
GRAM-POSITIVE BACTERIA
Peptidoglycan layer of cell wall Insertion of MAC into bacterial Streptococcus
membrane is prevented by thick
layer of peptidoglycan
Bacterial capsule Capsule provides physical barrier Streptococcus pneumoniae
between C3b deposited on
bacterial membrane and CR1
on phagocytic cells
OTHER MICROBES
Proteins that mimic complement Protein present in various bacteria, Vaccinia virus, herpes simplex,
regulatory proteins viruses, fungi, and protozoans Epstein-Barr virus, Trypanosoma
inhibit the complement cascade cruzi, Candida albicans
KEY: CR1 H11005 type 1 complement receptor; LPS H11005 lipopolysaccharide; MAC H11005 membrane-attack complex (C5b–9).
damage and restore its osmotic stability. An unfortunate con-
sequence of this effect is that complement-mediated lysis by
antibodies specific for tumor-cell antigens, which offers a po-
tential weapon against cancer, may be rendered ineffective by
endocytosis of the MAC (see Chapter 22).
Cleavage Products of Complement
Components Mediate Inflammation
The complement cascade is often viewed in terms of the fi-
nal outcome of cell lysis, but various peptides generated
during formation of the MAC play a decisive role in the de-
velopment of an effective inflammatory response (see Table
13-3). The smaller fragments resulting from complement
cleavage, C3a, C4a, and C5a, called anaphylatoxins, bind to
receptors on mast cells and blood basophils and induce de-
granulation, with release of histamine and other pharmaco-
logically active mediators. The anaphylatoxins also induce
smooth-muscle contraction and increased vascular perme-
ability. Activation of the complement system thus results in
influxes of fluid that carries antibody and phagocytic cells
to the site of antigen entry. The activities of these highly re-
active anaphylatoxins are regulated by a serum protease
called carboxypeptidase N, which cleaves an Arg residue
from the C terminus of the molecules, yielding so-called
des-Arg forms. The des-Arg forms of C3a and C4a are com-
pletely inactive while that of C5a retains about 10% of its
chemotactic activity and 1% of its ability to cause smooth
muscle contraction.
C3a, C5a, and C5b67 can each induce monocytes and
neutrophils to adhere to vascular endothelial cells, ex-
travasate through the endothelial lining of the capillary, and
migrate toward the site of complement activation in the tis-
sues. C5a is most potent in mediating these processes, effec-
tive in picomolar quantities. The role of complement in
leukocyte chemotaxis is discussed more fully in Chapter 15.
C3b and C4b Binding Facilitates
Opsonization
C3b is the major opsonin of the complement system, al-
though C4b and iC3b also have opsonizing activity. The am-
plification that occurs with C3 activation results in a coating
of C3b on immune complexes and particulate antigens.
Phagocytic cells, as well as some other cells, express comple-
ment receptors (CR1, CR3, and CR4) that bind C3b, C4b, or
iC3b (see Table 13-4). Antigen coated with C3b binds to cells
bearing CR1. If the cell is a phagocyte (e.g., a neutrophil,
monocyte, or macrophage), phagocytosis will be enhanced
(Figure 13-12). Activation of phagocytic cells by various
agents, including C5a anaphylatoxin, has been shown to in-
crease the number of CR1s from 5000 on resting phagocytes
to 50,000 on activated cells, greatly facilitating their phagocy-
tosis of C3b-coated antigen. Recent studies indicate that
complement fragment C3b acts as an adjuvant when coupled
with protein antigens. C3b targets the antigen directly to the
phagocyte, enhancing the initiation of antigen processing
and accelerating specific antibody production.
314 PART III Immune Effector Mechanisms
FIGURE 13-12 (a) Schematic representation of the roles of C3b
and antibody in opsonization. (b) Electron micrograph of Epstein-
Barr virus coated with antibody and C3b and bound to the Fc and
C3b receptor (CR1) on a B lymphocyte. [Part (b) from N. R. Cooper
and G. R. Nemerow, 1986, in Immunobiology of the Complement
System, Academic Press.]
Bacterium
Complement
activation
Fc receptor
CR1
C3b
Phagocytosis
Nucleus
IgG
(a)
Phagocyte
(b)
Coated
virus
particle
The Complement System Also
Neutralizes Viral Infectivity
For most viruses, the binding of serum antibody to the re-
peating subunits of the viral structural proteins creates par-
ticulate immune complexes ideally suited for complement
activation by the classical pathway. Some viruses (e.g., retro-
viruses, Epstein-Barr virus, Newcastle disease virus, and
rubella virus) can activate the alternative, lectin, or even the
classical pathway in the absence of antibody.
The complement system mediates viral neutralization by
a number of mechanisms. Some degree of neutralization is
achieved through the formation of larger viral aggregates,
simply because these aggregates reduce the net number of in-
fectious viral particles. Although antibody plays a role in the
formation of viral aggregates, in vitro studies show that the
C3b component facilitates aggregate formation in the pres-
ence of as little as two molecules of antibody per virion. For
example, polyoma virus coated with antibody is neutralized
when serum containing activated C3 is added.
The binding of antibody and/or complement to the sur-
face of a viral particle creates a thick protein coating that can
be visualized by electron microscopy (Figure 13-13). This
coating neutralizes viral infectivity by blocking attachment
to susceptible host cells. The deposits of antibody and com-
plement on viral particles also facilitate binding of the viral
particle to cells possessing Fc or type 1 complement receptors
(CR1). In the case of phagocytic cells, such binding can be
followed by phagocytosis and intracellular destruction of the
ingested viral particle. Finally, complement is effective in
lysing most, if not all, enveloped viruses, resulting in frag-
mentation of the envelope and disintegration of the nucleo-
capsid.
The Complement System Clears Immune
Complexes from Circulation
The importance of the complement system in clearing im-
mune complexes is seen in patients with the autoimmune dis-
ease systemic lupus erythematosus (SLE). These individuals
produce large quantities of immune complexes and suffer tis-
sue damage as a result of complement-mediated lysis and the
induction of type II or type III hypersensitivity (see Chapter
16). Although complement plays a significant role in the devel-
opment of tissue damage in SLE, the paradoxical finding is
that deficiencies in C1, C2, C4, and CR1 predispose an indi-
vidual to SLE; indeed, 90% of individuals who completely lack
C4 develop SLE. The complement deficiencies are thought to
interfere with effective solubilization and clearance of immune
complexes; as a result, these complexes persist, leading to tissue
damage by the very system whose deficiency was to blame.
The coating of soluble immune complexes with C3b is
thought to facilitate their binding to CR1 on erythrocytes. Al-
though red blood cells express lower levels of CR1 (~5 H11003 10
2
per cell) than granulocytes do (~5 H11003 10
4
per cell), there are
about 10
3
red blood cells for every white blood cell; therefore,
erythrocytes account for about 90% of the CR1 in the blood.
For this reason, erythrocytes play an important role in binding
C3b-coated immune complexes and carrying these complexes
to the liver and spleen. In these organs, immune complexes are
stripped from the red blood cells and are phagocytosed,
thereby preventing their deposition in tissues (Figure 13-14).
In SLE patients, deficiencies in C1, C2, and C4 each contribute
to reduced levels of C3b on immune complexes and hence in-
hibit their clearance. The lower levels of CR1 expressed on the
erythrocytes of SLE patients also may interfere with the proper
binding and clearance of immune complexes.
The Complement System CHAPTER 13 315
FIGURE 13-13 Electron micrographs of negatively stained prepara-
tions of Epstein-Barr virus. (a) Control without antibody. (b) Antibody-
coated particles. (c) Particles coated with antibody and complement.
[From N. R. Cooper and G. R. Nemerow, 1986,inImmunobiology of the
Complement System, Academic Press.]
(a) (b) (c)
complex diseases, individuals with such complement deficien-
cies may suffer from recurrent infections by pyogenic (pus-
forming) bacteria such as streptococci and staphylococci. These
organisms are gram-positive and therefore resistant to the lytic
effects of the MAC. Nevertheless, the early complement com-
ponents ordinarily prevent recurrent infection by mediating a
localized inflammatory response and opsonizing the bacteria.
Deficiencies in factor D and properdin—early components of
the alternative pathway—appear to be associated with Neisseria
infections but not with immune-complex disease.
Patients with C3 deficiencies have the most severe clinical
manifestations, reflecting the central role of C3 in activation
of C5 and formation of the MAC. The first patient identified
with a C3 deficiency was a child suffering from frequent se-
vere bacterial infections and initially diagnosed as having
agammaglobulinemia. After tests revealed normal immuno-
globulin levels, a deficiency in C3 was discovered. This case
highlights the critical function of the complement system in
converting a humoral antibody response into an effective de-
fense mechanism. The majority of patients with C3 deficiency
have recurrent bacterial infections and may have immune-
complex diseases.
Individuals with homozygous deficiencies in the compo-
nents involved in the MAC develop recurrent meningococcal
and gonococcal infections caused by Neisseria species. In
normal individuals, these gram-negative bacteria are gener-
ally susceptible to complement-mediated lysis or are cleared
by the opsonizing activity of C3b. MAC-deficient individuals
rarely have immune-complex disease, which suggests that
they produce enough C3b to clear immune complexes. Inter-
estingly, a deficiency in C9 results in no clinical symptoms,
suggesting that the entire MAC is not always necessary for
complement-mediated lysis.
Congenital deficiencies of complement regulatory proteins
have also been reported. The C1 inhibitor (C1Inh) regulates
activation of the classical pathway by preventing excessive C4
and C2 activation by C1. Deficiency of C1Inh is an autosomal
dominant condition with a frequency of 1 in 1000. The defi-
ciency gives rise to a condition called hereditary angioedema,
which manifests clinically as localized edema of the tissue, of-
ten following trauma, but sometimes with no known cause.
The edema can be in subcutaneous tissues or within the bowel,
where it causes abdominal pain, or in the upper respiratory
tract, where it causes obstruction of the airway.
Studies in humans and experimental animals with ho-
mozygous deficiencies in complement components have
316 PART III Immune Effector Mechanisms
Phagocyte
BLOOD
Ig
Ag
Soluble immune complex
Complement activation
C3b
CR1
Erythrocyte
LIVER
AND
SPLEEN
FIGURE 13-14 Clearance of circulating immune complexes by reac-
tion with receptors for complement products on erythrocytes and re-
moval of these complexes by receptors on macrophages in the liver
and spleen. Because erythrocytes have fewer receptors than macro-
phages, the latter can strip the complexes from the erythrocytes as they
pass through the liver or spleen. Deficiency in this process can lead to
renal damage due to accumulation of immune complexes.
Complement Deficiencies
Genetic deficiencies have been described for each of the com-
plement components. Homozygous deficiencies in any of the
early components of the classical pathway (C1q, C1r, C1s, C4,
and C2) exhibit similar symptoms, notably a marked increase
in immune-complex diseases such as systemic lupus erythe-
matosus, glomerulonephritis, and vasculitis. These deficiencies
highlight the importance of the early complement reactions in
generating C3b, and the critical role of C3b in solubilization
and clearance of immune complexes. In addition to immune-
been the major source of information concerning the role of
individual complement components in immunity. These
findings have been greatly extended by studies using knock-
out mice genetically engineered to lack expression of specific
complement components. Investigations of in vivo comple-
ment activity in these animals has allowed dissection of the
complex system of complement proteins and the assignment
of precise biologic roles to each.
SUMMARY
a73
The complement system comprises a group of serum pro-
teins, many of which exist in inactive forms.
a73
Complement activation occurs by the classical, alternative,
or lectin pathways, each of which is initiated differently.
a73
The three pathways converge in a common sequence of
events that leads to generation of a molecular complex that
causes cell lysis.
a73
The classical pathway is initiated by antibody binding to a
cell target; reactions of IgM and certain IgG subclasses ac-
tivate this pathway.
a73
Activation of the alternative and lectin pathways is anti-
body-independent. These pathways are initiated by reac-
tion of complement proteins with surface molecules of
microorganisms.
a73
In addition to its key role in cell lysis, the complement sys-
tem mediates opsonization of bacteria, activation of in-
flammation, and clearance of immune complexes.
a73
Interactions of complement proteins and protein frag-
ments with receptors on cells of the immune system con-
trol both innate and acquired immune responses.
a73
Because of its ability to damage the host organism, the
complement system requires complex passive and active
regulatory mechanisms.
a73
Clinical consequences of inherited complement deficien-
cies range from increases in susceptibility to infection to
tissue damage caused by immune complexes.
References
Ahearn, J. M., and D. T. Fearon. 1989. Structure and function of
the complement receptors CR1 (CD35) and CR2 (CD21). Adv.
Immunol. 46:183.
Carroll, M. C. 2000. The role of complement in B-cell activation
and tolerance. Adv. Immunol. 74:61.
Laurent, J., and M. T. Guinnepain. 1999. Angioedema associated
with C1 inhibitor deficiency. Clin. Rev. Allergy. & Immunol.
17:513.
Lindahl, G., U. Sjobring, and E. Johnsson. 2000. Human comple-
ment regulators: a major target for pathogenic microorgan-
isms. Curr. Opin. Immunol. 12:44.
Lokki, M. L., and H. R. Colten. 1995. Genetic deficiencies of
complement. Ann. Med. 27:451.
Matsumoto, M., et al. 1997. Abrogation of the alternative com-
plement pathway by targeted deletion of murine factor B. Proc.
Natl. Acad. Sci. U.S.A. 94:8720.
Molina, H., and V. M. Holers. 1996. Markedly impaired humoral
immune response in mice deficient in complement receptors 1
and 2. Proc. Natl. Acad. Sci. U.S.A. 93:3357.
Muller-Eberhard, H. J. 1988. Molecular organization and
function of the complement system. Annu. Rev. Biochem.
57:321.
Nielsen, C. H., E. M. Fischer, and R. G. Q. Leslie. 2000.The role
of complement in the acquired immune response. Immunol-
ogy 100:4.
Nonaka, M. 2000. Origin and evolution of the complement sys-
tem. Curr. Top. Microbiol. Immunol. 248:37.
Pickering, M. C., and M. J. Walport. 2000. Links between com-
plement abnormalities and system lupus erythematosus.
Rheumatology 39:133.
Rautemaa, R., and S. Meri. 1999. Complement-resistance mech-
anisms of bacteria. Microbes and Infection/Institut Pasteur
1:785.
Sloand, E. M., et al. 1998. Correction of the PNH Defect by GPI-
anchored protein transfer. Blood 92:4439.
Turner, M. W. 1998. Mannose-binding lectin (MBL) in health
and disease. Immunobiol. 199:327.
USEFUL WEB SITES
http://www.complement-genetics.uni-mainz.de/
The Complement Genetics Homepage from the University of
Mainz gives chromosomal locations and information on ge-
netic deficiencies of complement proteins.
http://www.cehs.siu.edu/fix/medmicro/cfix.htm
A clever graphic representation of the basic assay for comple-
ment activity using red blood cell lysis, from D. Fix at Univer-
sity of Southern Illinois, Carbondale.
http://www.gla.ac.uk/Acad/Immunology/compsyst.htm
Notes from D. F. Lappin at University of Glasgow, UK, on the
complement system. The site includes a listing of all comple-
ment proteins and their molecular properties.
Study Questions
CLINICAL FOCUS QUESTION Explain why complement disorders
involving regulatory components such as PNH may be more se-
rious than deficiencies in the active complement components.
1. Indicate whether each of the following statements is true or
false. If you think a statement is false, explain why.
a. A single molecule of bound IgM can activate the C1q com-
ponent of the classical complement pathway.
The Complement System CHAPTER 13 317
Go to www.whfreeman.com/immunology Self-Test
Review and quiz of key terms
b. C3a and C3b are fragments of C3.
c. The C4 and C2 complement components are present in
the serum in a functionally inactive proenzyme form.
d. Nucleated cells tend to be more resistant to complement-
mediated lysis than red blood cells.
e. Enveloped viruses cannot be lysed by complement be-
cause their outer envelope is resistant to pore formation
by the membrane-attack complex.
f. C4-deficient individuals have difficulty eliminating im-
mune complexes.
2. Explain why serum IgM cannot activate complement by itself.
3. Would you expect a C1 or C3 complement deficiency to be
more serious clinically? Why?
4. Some microorganisms produce enzymes that can degrade the
Fc portion of antibody molecules. Why would such enzymes
be advantageous for the survival of microorganisms that pos-
sess them?
5. Complement activation can occur via the classical, alterna-
tive, or lectin pathway.
a. How do the three pathways differ in the substances that
can initiate activation?
b. Which portion of the overall activation sequence differs
in the three pathways? Which portion is similar?
c. How do the biological consequences of complement acti-
vation via these pathways differ?
6. Enucleated cells, such as red blood cells, are more susceptible
to complement-mediated lysis than nucleated cells.
a. Explain why the red blood cells of an individual are not
normally destroyed as the result of innocent-bystander ly-
sis by complement.
b. Under what conditions might complement cause lysis of
an individual’s own red blood cells?
7. Briefly explain the mechanism of action? of the following
complement regulatory proteins. Indicate which pathway(s)
each protein regulates.
a. C1 inhibitor (C1Inh)
b. C4b-binding protein (C4bBP)
c. Homologous restriction factor (HRF)
d. Decay-accelerating factor (DAF)
e. Factor H
f. Membrane cofactor protein (MCP)
8. For each complement component(s) or reaction (a–l), select
the most appropriate description listed below (1–13). Each
description may be used once, more than once, or not at all.
Complement Component(s)/Reactions
a. ______C3b
b. ______C1, C4, C2, and C3
c. ______C9
d. ______C3, factor B, and factor D
e. ______C1q
f. ______C4b2a3b
g. ______C5b, C6, C7, C8, and C9
h. ______C3 → C3a H11001 C3b
i. ______C3a, C5a, and C5b67
j. ______C3a, C4a, and C5a
k. ______C4b2a
l. ______C3b H11001 B → C3bBb H11001 Ba
Descriptions
(1) Reaction that produces major amplification during
activation
(2) Are early components of alternative pathway
(3) Compose the membrane-attack complex
(4) Mediates opsonization
(5) Are early components of classical pathway
(6) Has perforin-like activity
(7) Binds to Fc region of antibodies
(8) Have chemotactic activity
(9) Has C3 convertase activity
(10) Induce degranulation of mast cells (are anaphylatoxins)
(11) Has C5 convertase activity
(12) Reaction catalyzed by factor D
(13) Reaction catalyzed by C1qr
2
s
2
9. You have prepared knockout mice with mutations in the
genes that encode various complement components. Each
knockout strain cannot express one of the complement
components listed across the top of the table below. Predict
the effect of each mutation on the steps in complement acti-
vation and on the complement effector functions indicated
in the table below using the following symbols: NE H11005 no
effect; D H11005 process/function decreased but not abolished;
A H11005 process/function abolished.
318 PART III Immune Effector Mechanisms
Component knocked out
C1qC4 C3 C5 C6 C9 Factor B
COMPLEMENT ACTIVATION
Formation of
C3 convertase
in classical
pathway
Formation of
C3 convertase
in alternative
pathway
Formation of
C5 convertase
in classical
pathway
Formation of
C5 convertase
in alternative
pathway
EFFECTOR FUNCTIONS
C3b-mediated
opsonization
Neutrophil
chemotaxis
Cell lysis