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Mainly IgM, low-affinity

antibodies; short-lived

plasma cells

T-independent

IgM Polysaccharide

antigen

Other signals

(e.g., complement

protein)

B-1 cells,

marginal

zone B cells

Fig. 7.2 T-dependent and T-independent antibody responses. Antibody responses to protein antigens

require T cell help, and the antibodies produced typically show isotype switching and are of high affinity.

Nonprotein (e.g., polysaccharide) antigens are able to activate B cells without T cell help. Most T-dependent

responses are made by follicular B cells, whereas marginal zone B cells and B-1 cells play greater roles in

T-independent responses. Ig, Immunoglobulin.

140 CHAPTER 7 Humoral Immune Responses

Low-level

antibody

production

Amount of antibody

Activated

B cells

Plasma

cells

Days after first antigen exposure Days after second antigen exposure

IgG

IgG

Primary response Secondary response

Lag after

immunization

Peak

response

Antibody

isotype

Antibody

affinity

Usually 5–10 days

Smaller

Usually IgM>IgG

Lower average affinity,

more variable

Usually 1–3 days

Larger

Relative increase in IgG and, under

certain situations, in IgA or IgE

(heavy-chain isotype switching)

Higher average affinity

(affinity maturation)

IgM

First

infection

Repeat

infection

A

B

0 5 10 >30 0 5 10 >30

Plasma cells

in peripheral

lymphoid tissues

Memory

B cell

Plasma cells

in bone marrow

Memory

Plasma cells B cell

in bone marrow

Primary

antibody response

Secondary

antibody response

Naive B cell

Fig. 7.3 Features of primary and secondary antibody responses. Primary and secondary antibody

responses differ in several respects, illustrated schematically in (A) and summarized in (B). In a primary

response, naive B cells in peripheral lymphoid tissues are activated to proliferate and differentiate into antibody-secreting plasma cells and memory cells. Some plasma cells may migrate to and survive in the bone

marrow for long periods. In a secondary response, memory B cells are activated to produce larger amounts of

antibodies, often with more heavy-chain class switching and affinity maturation. These features of secondary

responses are seen mainly in responses to protein antigens, because these changes in B cells are stimulated

by helper T cells, and only proteins activate T cells (not shown). The kinetics of the responses may vary with

different antigens and types of immunization. Ig, Immunoglobulin.

CHAPTER 7 Humoral Immune Responses 141

With this introduction, we now discuss B cell activation and antibody production, beginning with the

responses of B cells to the initial encounter with antigen.

STIMULATION OF B LYMPHOCYTES BY

ANTIGEN

Humoral immune responses are initiated when antigen-specific B lymphocytes in the spleen, lymph nodes,

and mucosal lymphoid tissues recognize antigens.

Some of the antigens in tissues or in the blood are transported to and concentrated in the B cell–rich follicles and

marginal zones of these peripheral lymphoid organs. In

lymph nodes, macrophages lining the subcapsular sinus

may capture antigens and take them to the adjacent follicles, where the bound antigens are displayed to B cells.

B lymphocytes specific for an antigen use their membrane-bound Ig as receptors that recognize the antigen

directly, without any need for processing of the antigen. B

cells are capable of recognizing the native antigen, so the

antibodies that are subsequently secreted (which have the

same specificity as the B cell antigen receptors) are able to

bind to the native microbe or microbial product.

The recognition of antigen triggers signaling pathways that initiate B cell activation. As with T lymphocytes, B cell activation also requires signals in addition

to antigen recognition, and many of these second signals are produced during innate immune reactions to

microbes. In the following sections, we describe the

mechanisms of B cell activation by antigen and other

stimuli, followed by a discussion of the functional consequences of antigen recognition.

Antigen-Induced Signaling in B Cells

Antigen-induced clustering of membrane Ig receptors

triggers biochemical signals that activate B cells (Fig.

7.4). The process of B lymphocyte activation is, in principle, similar to the activation of T cells (see Chapter 5, Fig.

5.9). In B cells, antigen receptor–mediated signal transduction requires the bringing together (cross-linking)

of two or more membrane Ig molecules. Antigen receptor cross-linking occurs when two or more antigen

molecules in an aggregate, or repeating epitopes of one

antigen molecule, bind to adjacent membrane Ig molecules of a B cell. Polysaccharides, lipids, and other

nonprotein antigens often contain multiple identical

epitopes in each molecule and are therefore able to bind

to numerous Ig receptors on a B cell at the same time.

Even protein antigens may be expressed in an array on

the surface of microbes and are thus able to cross-link

antigen receptors of a B cell.

Signals initiated by antigen receptor cross-linking

are transduced by receptor-associated proteins. Membrane IgM and IgD, the antigen receptors of naive B

lymphocytes, have highly variable extracellular antigen-binding regions (see Chapter 4). However, these

membrane receptors have short cytoplasmic tails, so

although they recognize antigens, they do not themselves transduce signals. The receptors are noncovalently associated with two proteins, called Iga and Igß,

to form the B cell receptor (BCR) complex, analogous

to the T cell receptor (TCR) complex of T lymphocytes.

The cytoplasmic domains of Iga and Igß each contain

a conserved immunoreceptor tyrosine-based activation

motif (ITAM), similar to those found in signaling subunits of many other activating receptors in the immune

system (e.g., CD3 and ? proteins of the TCR complex;

see Chapter 5). When two or more antigen receptors

of a B cell are brought together by antigen-induced

cross-linking, the tyrosines in the ITAMs of Iga and Igß

are phosphorylated by tyrosine kinases associated with

the BCR complex. These phosphotyrosines recruit the

Syk tyrosine kinase (equivalent to ZAP-70 in T cells),

which is activated and in turn phosphorylates tyrosine

residues on adaptor proteins. These phosphorylated

proteins then recruit and activate a number of downstream molecules, mainly enzymes that initiate signaling cascades that activate transcription factors.

The net result of receptor-induced signaling in B

cells is the activation of transcription factors that switch

on the expression of genes whose protein products are

involved in B cell proliferation and differentiation. Some

of the important proteins are described below.

Role of Innate Immune Signals in B Cell

Activation

B lymphocytes express a receptor for a complement

system protein that provides second signals for the

activation of these cells (Fig. 7.5A). The complement

system, introduced in Chapter 2, is a collection of plasma

proteins that are activated by microbes and by antibodies

attached to microbes and function as effector mechanisms

of host defense (see Chapter 8). When the complement

system is activated by a microbe as part of the innate

immune response, the microbe becomes coated with

proteolytic fragments of the most abundant complement

142 CHAPTER 7 Humoral Immune Responses

protein, C3. One of these fragments is called C3d. B lymphocytes express a receptor for C3d called complement

receptor type 2 (CR2, or CD21). B cells that are specific

for a microbe’s antigens recognize the antigens by their

BCRs and simultaneously recognize the bound C3d via

the CR2 receptor. Engagement of CR2 greatly enhances

antigen-dependent activation responses of B cells by

enhancing tyrosine phosphorylation of ITAMs. This role

of complement in humoral immune responses illustrates

the fundamental tenet of the two-signal hypothesis that

was introduced in Chapter 2, that microbes or innate

immune responses to microbes provide signals in addition to antigen that are necessary for lymphocyte activation. In humoral immunity, complement activation

represents one way in which innate immunity facilitates

B lymphocyte activation.

Syk

P

P

P

P

P

Cross-linking

of membrane

Ig by antigen

Tyrosine

phosphorylation

events

Biochemical

intermediates

Active

enzymes

Transcription

factors

Adaptor

proteins

Myc NFAT NF-?B AP-1

Iga

mIg

Microbe

Igß

ITAM

Lyn

Fyn

Blk

PLC?2

activation

GTP/GDP

exchange on

Ras, Rac

Diacylglycerol

(DAG)

PKC ERK, JNK

P

Ras•GTP,

Rac•GTP

Inositol trisphosphate,

increased cytosolic Ca2+

Ca2+-dependent enzymes

Btk

Fig. 7.4 Antigen receptor–mediated signal transduction in B lymphocytes. Cross-linking of antigen

receptors on B cells by antigen triggers biochemical signals that are transduced by the immunoglobulin

(Ig)-associated proteins Iga and Igß. These signals induce early tyrosine phosphorylation events, activation

of various biochemical intermediates and enzymes, and activation of transcription factors. Similar signaling

events are seen in T cells after antigen recognition. Note that maximal signaling requires cross-linking of at

least two Ig receptors by antigens. AP-1, Activating protein 1; GDP, guanosine diphosphate; GTP, guanosine

triphosphate; ITAM, immunoreceptor tyrosine-based activation motif; NFAT, nuclear factor of activated T

cells; NF-?B, nuclear factor ?B; PKC, protein kinase C; PLC, phospholipase C.

CHAPTER 7 Humoral Immune Responses 143

Microbial products also directly activate B cells by

engaging innate pattern recognition receptors (see

Fig. 7.5B). B lymphocytes, similar to dendritic cells and

other leukocytes, express numerous Toll-like receptors

(TLRs; see Chapter 2). Pathogen-associated molecular

patterns bind to TLRs on the B cells, which triggers activating signals that work in concert with signals from the

antigen receptor. This combination of signals stimulates

B cell proliferation, differentiation, and Ig secretion,

thus promoting antibody responses against microbes.

Functional Consequences of B Cell Activation

by Antigen

B cell activation by multivalent antigen (and other signals) may initiate the proliferation and differentiation