What is CLONAL SELECTION THEORY?

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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 an 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:

1. Each lymphocyte bears a single type of receptor with a unique specificity (by V(D)J recombination).
2. Receptor occupation is required for cell activation.
3. The differentiated effector cells derived from an activated lymphocyte will bear receptors of identical specificity as the parental cell.
4. Those lymphocytes bearing receptors for self molecules will be deleted at an early stage.