Clonal selection theory

Clonal selection theory is a scientific theory in immunology that explains the functions of cells (lymphocytes) of the immune system in response to specific antigens invading the body. The concept was introduced by the Australian doctor Frank Macfarlane Burnet in 1957, in an attempt to explain the formation of a diversity of antibodies during initiation of the immune response. The theory has become a widely accepted model for how the immune system responds to infection and how certain types of B and T lymphocytes are selected for destruction of specific antigens.

The theory states that in a pre-existing group of lymphocytes (specifically B cells), a specific antigen only activates (i.e. selection) its counter-specific cell so that particular cell is induced to multiply (producing its clones) for antibody production. This activation occurs in secondary lymphoid organs such as the spleen and the lymph nodes. In short the theory is an explanation of the mechanism for the generation of diversity of antibody specificity. The first experimental evidence came in 1958, when Gustav Nossal and Joshua Lederberg showed that one B cell always produces only one antibody. The idea turned out to be the foundation of molecular immunology, especially in adaptive immunity.

The clonal selection theory can be summarised with the following four tenets:

In 1900, Paul Ehrlich proposed the so-called side chain theory of antibody production. According to it, certain cells exhibit on their surface different "side chains" (i.e. membrane-bound antibodies) able to react with different antigens. When an antigen comes, it binds to a matching side chain. Then the cell stops producing all other side chains and starts intensive synthesis and secretion of the antigen-binding side chain as a soluble antibody. This was a selection (though not clonal selection) theory far more accurate than the instructive theories that dominated immunology in the next decades.

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