Immunoglobulin E (IgE) antibodies activate mast

cell and eosinophil–mediated reactions that provide

defense against helminthic parasites and are involved

in allergic diseases. Most helminths are too large to be

phagocytosed, and their thick integument makes them

resistant to many of the microbicidal substances produced by neutrophils and macrophages. The humoral

immune response to helminths is dominated by IgE

antibodies. IgE binds to the worms and promotes the

attachment of eosinophils through the high-affinity Fc

receptor for IgE, FceRI, which is expressed on eosinophils and mast cells. Engagement of FceRI, together

with the cytokine interleukin-5 (IL-5) produced by Th2

helper T cells reacting against the helminths, leads to

activation of the eosinophils, which release their granule contents, including proteins that can kill the worms

(Fig. 8.8). IgE antibodies also bind to and activate mast

cells, which secrete cytokines, including chemokines,

that attract more leukocytes that function to destroy the

helminths.

This IgE-mediated reaction illustrates how Ig isotype

switching optimizes host defense. B cells respond to

helminths by switching to IgE, which is useful against

helminths, but B cells respond to most bacteria and

viruses by switching to IgG antibodies, which promote

phagocytosis by Fc?RI. As discussed in Chapters 6 and 7,

these patterns of isotype switching are determined by

the cytokines produced by helper T cells responding to

the different types of microbes.

IgE antibodies also are involved in allergic diseases

(see Chapter 11).

THE COMPLEMENT SYSTEM

The complement system is a collection of circulating

and cell membrane proteins that play important roles

in host defense against microbes and in antibodymediated tissue injury. The term complement refers to

the ability of these proteins to assist, or complement, the

activity of antibodies in destroying (lysing) cells, including microbes. The complement system may be activated

by microbes in the absence of antibody, as part of the

IgG Surface

antigen

NK cell

Low-affinity

Fc?RIII (CD16)

Killing of

antibodycoated cell

Antibodycoated cell

Fig. 8.7 Antibody-dependent cellular cytotoxicity. Antibodies of certain immunoglobulin G (IgG) subclasses (IgG1 and

IgG3) bind to antigens on the surface of infected cells, and

their Fc regions are recognized by an Fc? receptor on natural

killer (NK) cells. The NK cells are activated and kill the antibody-coated cells. Ig, Immunoglobulin.

Eosinophil

activation

Helminth

death

Helminth

FceRI IgE

Eosinophil

granule

contents

Eosinophil

Th2 cell

IL-5

Fig. 8.8 Immunoglobulin E (IgE)- and eosinophil-mediated

killing of helminths. IgE antibody binds to helminths and

recruits and activates eosinophils via FceRI, leading to degranulation of the cells and release of toxic mediators. Interleukin-5

(IL-5) secreted by Th2 cells enhances the ability of eosinophils

to kill the parasites. Ig, Immunoglobulin.

CHAPTER 8 Effector Mechanisms of Humoral Immunity 165

innate immune response to infection, and by antibodies

attached to microbes, as part of adaptive immunity (see

Fig. 2.12 in Chapter 2).

The activation of the complement system involves

sequential proteolytic cleavage of complement proteins,

leading to the generation of effector molecules that participate in eliminating microbes in different ways. This cascade of complement protein activation, like all enzymatic

cascades, is capable of achieving tremendous amplification, because even a small number of activated complement molecules produced early in the cascade may

generate a large number of effector molecules. Activated

complement proteins become covalently attached to the

cell surfaces where the activation occurs, ensuring that

complement effector functions are limited to the correct

sites. Normal host cells possess several regulatory mechanisms that inhibit the activation of complement and the

deposition of activated complement proteins, thus preventing complement-mediated damage to healthy cells.

Pathways of Complement Activation

There are three major pathways of complement activation: the alternative and lectin pathways are initiated by microbes in the absence of antibody, and the

classical pathway is initiated by certain isotypes of

antibodies attached to antigens (Fig. 8.9). Several proteins in each pathway interact in a precise sequence. The

most abundant complement protein in the plasma, C3,

plays a central role in all three pathways. The early steps

of all three pathways function to generate a large number of functionally active fragments of C3 bound to the

microbe or cell where the complement pathway was initiated. (By convention, the smaller proteolyic fragment

of any complement protein is given the “a” suffix, and

the larger piece is the “b” fragment; C2 is an exception.)

• The alternative pathway of complement activation is

triggered by spontaneous hydrolysis of C3 in plasma

at a low level. The breakdown products of C3 are

unstable, and, in the absence of infection, are rapidly degraded and lost. However, when a breakdown

product of C3 hydrolysis, called C3b, is deposited

on the surface of a microbe, it forms stable covalent

bonds with microbial proteins or polysaccharides.

The microbe-bound C3b binds another protein called

Factor B, which is then cleaved by a plasma protease

called Factor D to generate the Bb fragment. This

fragment remains attached to C3b, and the C3bBb

complex functions as a proteolytic enzyme, called

the alternative pathway C3 convertase, that breaks

down more C3. The C3 convertase is stabilized by

properdin, a positive regulator of the complement

system. As a result of this enzymatic activity, many

more C3b and C3bBb molecules are produced and

become attached to the microbe. Some of the C3bBb

molecules bind an additional C3b molecule, and the

resulting C3bBb3b complexes function as C5 convertases, to cleave the complement protein C5 and initiate the late steps of complement activation.

• The classical pathway of complement activation

is triggered when IgM or certain subclasses of IgG

(IgG1 and IgG3 in humans) bind to antigens (e.g., on

a microbial cell surface). As a result of this binding,

adjacent Fc regions of the antibodies become accessible to and bind the C1 complement protein (which

is made up of a binding component called C1q and

two proteases called C1r and C1s). The attached C1

becomes enzymatically active, resulting in the binding and sequential cleavage of two proteins, C4 and

C2. One of the C4 fragments that is generated, C4b,

becomes covalently attached to the antibody or to the

microbial surface where the antibody is bound, and

then binds C2, which is cleaved by active C1 to yield

the C4b2a complex. This complex is the classical pathway C3 convertase, which functions to break down

C3, and the C3b that is generated again becomes

attached to the microbe. Some of the C3b binds to

the C4b2a complex, and the resultant C4b2a3b complex functions as a C5 convertase, which cleaves the

C5 complement protein.

• The lectin pathway of complement activation is initiated not by antibodies but by the attachment of

plasma mannose-binding lectin (MBL) to microbes.

Serine proteases structurally related to C1s of the

classical pathway are associated with MBL and serve

to activate C4. The subsequent steps are essentially

the same as in the classical pathway.

The net result of these early steps of complement

activation is that microbes acquire a coat of covalently attached C3b. Note that the alternative and lectin

pathways are effector mechanisms of innate immunity,

whereas the classical pathway is a mechanism of adaptive

humoral immunity. These pathways differ in their initiation, but once triggered, their late steps are the same.

The late steps of complement activation are initiated

by the binding of C5 to the C5 convertase and subsequent proteolysis of C5, generating C5b (Fig. 8.10).

166 CHAPTER 8 Effector Mechanisms of Humoral Immunity

Alternative

Pathway

Classical

Pathway

Formation

of C3

convertase

Cleavage

of C3

Covalent

binding of

C3b;

formation

of C5

convertase

Binding of

complement

proteins to

microbial cell

surface or

antibody

C1

C3

convertase

C3

convertase

IgG antibody

C5

convertase

C3a C3a C3a

C5

convertase

Lectin

Pathway

Mannosebinding

lectin

Mannose

Late steps of complement activation

A

C5

convertase

C3

convertase

Microbe

C4 C2 C4 C2

C3

C3

C5

C3b

C3b

C3b

C3b

Bb

C3

C3bBb

C4b C4b

C4b

C4b 2a C4b 2a

C4b

2a 2a

2a

C3

C4b2a

C3b C3b

C5a

C5b

C5a

C5b

C5a

C5b

2a

C5 C5

C3b C3b C3b Bb C4b2a

Fig. 8.9 Early steps of complement activation. A, The steps in the activation of the alternative, classical,

and lectin pathways. Although the sequence of events is similar, the three pathways differ in their requirement for antibody and the proteins used. Note that C5 is cleaved by the C5 convertase but is not a component

of the enzyme.

CHAPTER 8 Effector Mechanisms of Humoral Immunity 167

C3

Factor B

Factor D

C3b binds to the surface of microbes,

where it functions as an opsonin and

as a component of C3 and

C5 convertases

C3a stimulates inflammation

Bb is a serine protease and the active

enzyme of C3 and C5 convertases

Plasma serine protease that cleaves

Factor B when it is bound to C3b

640–1660

200

1–2

C1

(C1qr2s2)

C4

C2

Initiates the classical pathway; C1q

binds to Fc portion of antibody; C1r

and C1s are proteases that lead to

C4 and C2 activation

C4b covalently binds to surfaces of

microbes or cells where antibody is

bound and complement is activated

C4b binds to C2 for cleavage by C1s

C4a stimulates inflammation

C2a is a serine protease functioning

as an active enzyme of C3 and

C5 convertases

150–450

20

Mannose

binding

lectin

(MBL)

Initiates the lectin pathway; MBL

binds to terminal mannose residues

of microbial carbohydrates. MBLassociated proteases activate C4 and

C2, as C1r and C1s do in the

classical pathway.

0.8–1

Protein Serum conc.

(µg/mL)

Function

Alternative pathway proteins

Protein Serum conc.

(µg/mL)

Function

Classical and lectin pathway proteins C

B

Fig. 8.9, cont’d B, The important properties of the proteins involved in the early steps of the alternative pathway of complement activation. C, The important properties of the proteins involved in the early steps of the

classical and lectin pathways. Note that C3, which is listed among the alternative pathway proteins (B), also

is the central component of the classical and lectin pathways.

168 CHAPTER 8 Effector Mechanisms of Humoral Immunity

C5a

C5

convertase

Poly-C9

C7 C8

Membrane attack

complex (MAC)

C5b

Inflammation

Cell

lysis

A

C5

C6

C7

C8 60

60

C5b initiates assembly of the

membrane attack complex (MAC)

C5a stimulates inflammation

Component of the MAC: binds to C5b

and accepts C7

Component of the MAC: binds C5b, 6

and inserts into lipid membranes

Component of the MAC: binds C5b, 6, 7

and initiates binding and polymerization

of C9

C9

80

45

90

Component of the MAC: binds C5b, 6,

7, 8 and polymerizes to form

membrane pores

B

C9

Plasma membrane

C7 C8

C5b C5

C3b Bb C3b

Protein Serum conc.

(µg/mL)

Function

C6 C7

C5b

C6 C7 C8

C5b

C6

C6

Fig. 8.10 Late steps of complement activation. A, The late steps of complement activation start after the

formation of the C5 convertase and are identical in the alternative and classical pathways. Products generated

in the late steps induce inflammation (C5a) and cell lysis (membrane attack complex). B, Properties of the

proteins in the late steps of complement activation.

The remaining components, C6, C7, C8, and C9, bind

sequentially to a complex nucleated by C5b. The final

protein in the pathway, C9, polymerizes to form a pore

in the cell membrane through which water and ions can

enter, causing death of the microbe. The C5-9 complex

is called the membrane attack complex (MAC), and its

formation is the end result of complement activation.

Functions of the Complement System

The complement system plays an important role in

the elimination of microbes during innate and adaptive immune responses. The main effector functions of

the complement system are illustrated in Fig. 8.11.

• Opsonization. Microbes coated with C3b are

phagocytosed by virtue of C3b being recognized by

CHAPTER 8 Effector Mechanisms of Humoral Immunity 169

complement receptor type 1 (CR1, or CD35), which

is expressed on phagocytes. Thus, C3b functions

as an opsonin. Opsonization is probably the most

important function of complement in defense against

microbes.

• Cell lysis. The MAC can induce osmotic lysis of cells,

including microbes. MAC-induced lysis is effective

only against microbes that have thin cell walls and

little or no glycocalyx, such as the Neisseria species of

bacteria.

• Inflammation. The small peptide fragments C3a and

C5a, which are produced by proteolysis of C3 and C5,

are chemotactic for neutrophils, stimulate the release

of inflammatory mediators from various leukocytes,

and stimulate movement of leukocytes and plasma

proteins across the endothelium into tissues. In this

way, complement fragments induce inflammatory

reactions that also serve to eliminate microbes.

In addition to its antimicrobial effector functions,

the complement system stimulates B cell responses

Opsonization and phagocytosis

Stimulation of inflammatory reactions

Binding of

C3b to microbe

(opsonization)

Recognition of

bound C3b by

phagocyte C3b receptor

Phagocytosis

and killing

of microbe

Destruction

of microbes

by leukocytes

Recruitment and

activation of

 leukocytes by

C5a and C3a

C3a,

C5a

Proteolysis of C3

and C5 to release

C3a and C5a

Activation of

C5 convertase

Formation of the

membrane attack

complex (MAC)

Osmotic lysis

of microbe

Microbe

Microbe

CR1

A

B

C

Complement-mediated cytolysis

Microbe

MAC

C3b

C3b