Characteristics of the Primary Response

A lag period of approximately 10 days to 2 weeks occurs before a substantial amount of antibody can be detected in the blood following the first (primary) exposure to an antigen (figure 16.11). During this delay, the individual could very well experience symptoms of an infection, which could be life-threatening. However, the immune system is actively responding; B cells and T cells that recognize the antigen interact directly and communicate with each other via cytokines. Clonal selection and expansion of both types of cells occurs, generating a population of cells

1) B-cell receptor binds to antigen

B-cell receptor

2) B cell internalizes antigen

1) B-cell receptor binds to antigen

Antigen fragment presented by MHC class II molecule

2) B cell internalizes antigen

T-cell receptor

Antigen fragment presented by MHC class II molecule

T-cell receptor

3) B cell degrades antigen into peptide fragments, then presents them in the groove of MHC class II molecules that reside on the B-cell surface.

Cytokine delivery

4) If the T-cell receptor of an effector T-helper cell binds to one of the fragments, then cytokines are delivered to the B cell, initiating the process of clonal expansion.

3) B cell degrades antigen into peptide fragments, then presents them in the groove of MHC class II molecules that reside on the B-cell surface.

Cytokine delivery

4) If the T-cell receptor of an effector T-helper cell binds to one of the fragments, then cytokines are delivered to the B cell, initiating the process of clonal expansion.

Figure 16.9 Antigen Presentation by a B Cell This process enlists the assistance of an effectorT-helper cell, which can activate the B cell, allowing it to undergo clonal expansion.The T-helper cell also directs affinity maturation, class switching, and the formation of memory cells.

that respond to the antigen. As some of the activated B cells continue dividing, others differentiate to form plasma cells, which begin producing IgM. The net result is the slow but steady increase in the titer, or concentration, of antibodies.

Some of the proliferating B cells undergo changes, enhancing the immune response over time. These include:

■ Affinity maturation. This is a form of natural selection that occurs among proliferating B cells, effectively fine-tuning the quality of the response with respect to antibody specificity (figure 16.12). An inordinately large number of mutations naturally occur in certain regions of the antibody genes as the activated B cells replicate their DNA in preparation for division. Some of the mutations result in alterations in the antigen-binding site of the antibody (and therefore the B-cell receptor). B cells that bind antigen most tightly and for the longest duration are most likely to proliferate; others undergo apoptosis.

■ Class switching. Under the direction of cytokines produced by effector T-helper cells, some B cells become programmed to produce antibodies other than IgM. This alteration changes the class of antibody produced by the plasma cells that descend from these B cells. Circulating B cells most commonly switch to IgG production (figure 16.13); B cells residing in the mucosal-associated lymphoid tissues

10 mm

Figure 16.10 Lymphocytes and Plasma Cells (a) Light micrograph of a T lymphocyte.The morphology is the same as that of a B lymphocyte. (b) Scanning electron micrograph of a T lymphocyte.T and B lymphocytes cannot be distinguished by microscopy. (c) Plasma cell, a form of B cell that is highly differentiated to produce large amounts of antibody. Note the extensive rough endoplasmic reticulum, the site of protein synthesis. All of the antibodies produced by a single plasma cell have the same specificity.

10 mm

10 mm

Figure 16.10 Lymphocytes and Plasma Cells (a) Light micrograph of a T lymphocyte.The morphology is the same as that of a B lymphocyte. (b) Scanning electron micrograph of a T lymphocyte.T and B lymphocytes cannot be distinguished by microscopy. (c) Plasma cell, a form of B cell that is highly differentiated to produce large amounts of antibody. Note the extensive rough endoplasmic reticulum, the site of protein synthesis. All of the antibodies produced by a single plasma cell have the same specificity.

16.6 B Lymphocytes and the Antibody Response 405

Nucleus Rough endoplasmic reticulum

16.6 B Lymphocytes and the Antibody Response 405

Nucleus Rough endoplasmic reticulum

(c)

10 mm most commonly switch to IgA production, ultimately providing mucosal immunity.

■ Formation of memory cells. Some of the B cells that have undergone class switching form memory cells. Memory B cells persist in the body for years and are present in numbers sufficient to give a prompt and effective secondary response when the same antigen is encountered again at a later time.

The antibody response begins to wane as the antibodies that accumulate clear the antigen. Progressively fewer molecules of antigen remain to stimulate the lymphocytes, and, as a result, the activated lymphocytes undergo apoptosis. Memory B cells, however, are long-lived even in the absence of antigen. In addition, specialized dendritic cells called follicular dendritic cells appear to retain antigen in small amounts, using it to continually rejuvenate the memory response. The process is not well understood.

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