Info

Peak response

Smaller

Larger

Antibody isotype

Usually IgM > IgG

Relative increase in IgG and, under certain situations, in IgA or IgE

Antibody affinity

Lower average affinity, more variable

Higher average affinity (affinity maturation)

Induced by

All immunogens

Only protein antigens

FIGURE 11-2 Primary and secondary humoral immune responses. In a primary immune response, naive B cells are stimulated by antigen, become activated, and differentiate into antibody-secreting cells that produce antibodies specific for the eliciting antigen. Some of the antibody-secreting plasma cells migrate to and survive in the bone marrow, where they continue to produce antibodies for long periods. Long-lived memory B cells are also generated during the primary response. A secondary immune response is elicited when the same antigen stimulates these memory B cells, leading to more rapid proliferation and differentiation and production of greater quantities of specific antibody than are produced in the primary response. The principal characteristics of primary and secondary antibody responses are summarized in the table. These features are typical of T cell-dependent antibody responses to protein antigens.

FIGURE 11-2 Primary and secondary humoral immune responses. In a primary immune response, naive B cells are stimulated by antigen, become activated, and differentiate into antibody-secreting cells that produce antibodies specific for the eliciting antigen. Some of the antibody-secreting plasma cells migrate to and survive in the bone marrow, where they continue to produce antibodies for long periods. Long-lived memory B cells are also generated during the primary response. A secondary immune response is elicited when the same antigen stimulates these memory B cells, leading to more rapid proliferation and differentiation and production of greater quantities of specific antibody than are produced in the primary response. The principal characteristics of primary and secondary antibody responses are summarized in the table. These features are typical of T cell-dependent antibody responses to protein antigens.

enter the conduits. These antigens may be captured in the medullary region by resident dendritic cells and transported into follicles, where they can activate B cells.

• Antigens in immune complexes may bind to complement receptors (in particular the complement receptor type 2 or CR2) on marginal zone B cells, and these cells can transfer the immune complex-containing antigens to follicular B cells.

• Antigen in immune complexes may also bind to CR2 on the surface of follicular dendritic cells and be presented to antigen-specific B cells.

• Blood-borne pathogens may be captured by plasmacytoid dendritic cells in the blood and transported to the spleen, where they may be delivered to marginal zone B cells.

• Polysaccharide antigens can be captured by macrophages in the marginal zone of splenic lymphoid follicles and displayed or transferred to B cells in this area.

In all these cases, the antigen that is presented to B cells is generally in its intact, native conformation and is not processed by antigen-presenting cells. This, of

Follicular B cells

Follicular B cells

Spleen, other lymphoid organs

IgD Protein antigen + helper T cell

igG igA igE

Germinal center reaction

IgD Protein antigen + helper T cell

T-dependent, isotype-switched, high-affinity antibodies; long-lived plasma cells igG igA igE

Germinal center reaction

<igM

Marginal A zone B cells

<igM

Polysaccharides, lipids, etc.

Marginal A zone B cells

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