clonal selection theory

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clonal selection theory A theory explaining how the cells of the immune system produce large quantities of the right antibody at the right time, i.e. when the appropriate antigen is encountered. It proposes that there is a pre-existing pool of lymphocytes (B cells) consisting of numerous small subsets. Each subset carries a unique set of surface antibody molecules with its own particular binding characteristics. If a cell encounters and binds the corresponding antigen it is `selected' – stimulated to divide repeatedly and produce a large clone of identical cells, all secreting the antibody. The involvement of helper T cells (see T cell) is essential for activation of the B cell. A form of clonal selection is also invoked to explain the development of immunological tolerance.