Clonal Selection Theory

The immunization procedures of Jenner and Pasteur were effective because the people who were inoculated produced a secondary rather than a primary response when exposed to the virulent pathogens. The type of protection they were afforded does not depend on accumulations of antibodies in the blood, since secondary responses occur even after antibodies produced by the primary response have disappeared. Immunizations, therefore, seem to produce a type of "learning" in which the ability of the immune system to combat a particular pathogen is improved by prior exposure.

The mechanisms by which secondary responses are produced are not completely understood; the clonal selection theory, however, accounts for most of the evidence. According to this theory, B lymphocytes inherit the ability to produce particular antibodies (and T lymphocytes inherit the ability to respond to particular antigens). A given B lymphocyte can produce only one type of antibody, with specificity for one antigen. Since this ability is genetically inherited rather than acquired, some lymphocytes can respond to smallpox, for example, and produce antibodies against it even if the person has never been previously exposed to this disease.

The inherited specificity of each lymphocyte is reflected in the antigen receptor proteins on the surface of the lymphocyte's plasma membrane. Exposure to smallpox antigens thus stimulates these specific lymphocytes to divide many times until a large population of genetically identical cells—a clone—is produced. Some of these cells become plasma cells that secrete antibodies for the primary response; others become memory cells that can be stimulated to secrete antibodies during the secondary response (fig. 15.23).

Notice that, according to the clonal selection theory (table 15.8), antigens do not induce lymphocytes to make the appropriate antibodies. Rather, antigens select lymphocytes (through interaction with surface receptors) that are already able to make antibodies against that antigen. This is analogous to evolution by natural selection. An environmental agent (in this case, antigens) acts on the genetic diversity already present in a population of organisms (lymphocytes) to cause an increase in number of the individuals selected.

First day

B lymphocyte

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X X

X

Ribosomes

Third day /

OC

X X

X

reticulum

Fourth day oc

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Fifth day /

Memo

Development of

y cel lone

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Plasma cell

■ Figure 15.23 The clonal selection theory as applied to B lymphocytes. Most members of the B lymphocyte clone become memory cells, but some antibody-secreting plasma cells.

Table 15.8 Summary of the Clonal Selection Theory (As Applied to B Cells)

Process

Results

Lymphocytes inherit the ability to produce specific antibodies.

Antigens interact with antibody receptors on the lymphocyte surface.

Subsequent exposure to the specific antigens produces a more efficient response.

Prior to antigen exposure, lymphocytes that can make the appropriate antibodies are already present in the body.

Antigen-antibody interaction stimulates cell division and the development of lymphocyte clones that contain memory cells and plasma cells that secrete antibodies.

Exposure of lymphocyte clones to specific antigens results in greater and more rapid production of specific antibodies.

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Responses

  • Saara Hull
    What is clonal selection theory as it applies to b cells?
    8 years ago
  • matias
    How is the clonal selection theory and the theory of natural selection related?
    8 years ago
  • donna
    What is the clonal selction theory as it applies to b cells?
    7 years ago

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