Clonal Selection of Lymphocytes

Early on, immunologists recognized that the immune system is capable of making a seemingly infinite array of antibody specificities, an ability that could potentially be explained by either of two general mechanisms. One hypothesis suggested that antibody-producing cells are able to alter the specificity of the antibodies in response to a particular antigen; in other words, the antigen is able to induce the proper fit. The other hypothesis, proposed in the 1950s, stated that each cell in a large population of antibody-producing cells makes only a single specificity of antibody molecule. Then, when antigen is introduced, only the cells that make the appropriate antibody can bind to the antigen, which promotes their multiplication; this process is called clonal selection (figure 16.8). Repeated cycles of cell division generate a population of copies, or clones, of the initial cell; this process is called clonal expansion.

The model of clonal selection and expansion, now called the clonal selection theory, has been shown to be a critical theme in adaptive immunity, pertaining to both B cells and T cells. As lymphocytes mature in the primary lymphoid organs, a population of cells able to recognize a nearly infinite variety of antigens is generated; each individual cell, however, is able to recognize and respond to only one epitope. Thus, if a person's immune system can make antibodies to billions of different epitopes, that person must have billions of different B cells, each B cell interacting with a single epitope. In fact, the body is estimated to have approximately 109 (1 billion) B cells, and only one or a few will recognize a given epitope. Since a pathogen has multiple different epitopes, a number of distinct B cells will recognize it. The process of generating the diversity in antigen recognition is random and does not require previous exposure to antigen; the mechanisms will be described later.

The mature lymphocytes residing in the secondary lym-phoid organs are waiting for the "antigen of their dreams," an antigen to which a particular lymphocyte is programmed to respond. When an antigen enters a lymphoid organ, only the lymphocytes that specifically recognize it may respond; the specificity of the antigen receptor they carry on their surface (B-cell receptor or T-cell receptor) governs this recognition. Lymphocytes that do not recognize the antigen remain quiescent. Recall that in most cases, lymphocytes that recognize antigen require accessory signals, a "second opinion" by another cell type, in order to respond. This provides a mechanism by which the immune system can avoid mounting a response against "self" molecules.

Some progeny of the lymphocytes that encountered their "dream antigen" may leave the secondary lymphoid organs and migrate to the tissues where they continue responding for as long as the antigen is present. Without sustained stimulation by antigen, these cells will undergo apoptosis, curtailing the immune response. ■ apoptosis, p. 387

The activities of individual lymphocytes change over the lifetime of the cell, particularly as the cell responds to antigen.

Nester-Anderson-Roberts: I III. Microorganisms and I 16. The Adaptive Immune I I © The McGraw-Hill

Microbiology, A Human Humans Response Companies, 2003

Perspective, Fourth Edition

Figure 16.8 Clonal Selection and Expansion During the Antibody Response

Naive B cells, each with different specific antigen receptors (B-cell receptors)

Figure 16.8 Clonal Selection and Expansion During the Antibody Response

B cell exposed to b antigen

16.5 Clonal Selection of Lymphocytes 403

dB-cell receptor on surface of B cell

Long-lived memory cell for antigen b tDV\

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