The mechanism whereby 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. 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 is essential for activation of the B cell. A similar process generates T cells with specificity towards certain antigens. A form of clonal selection is also invoked to explain the development of immunological tolerance.