The immune system has traditionally been divided into innate and adaptive components, each with a different function and role. The adaptive component is organized around two classes of specialized cells, T cells and B cells. Since each lymphocyte displays a single kind of structurally unique receptor, the repertoire of antigen receptors in the entire population of lymphocytes is very large and extremely diverse. The size and diversity of this repertoire increase the probability that an individual lymphocyte will encounter an antigen that binds to its receptor, thereby triggering activation and proliferation of the cell. This process, termed clonal selection, accounts . . .