THE immune response is initiated by the introduction of a foreign molecule, an antigen, into an organism. The manner in which an antigen stimulates immunity is immensely complex — a reflection, no doubt, of the multiplicity of effector functions that must be generated and of the fine balance that must develop between what is prophylactic (and hence beneficial to the organism) and what is harmful. More specifically, the immune cells must first identify the antigen as foreign and precisely discriminate this foreign molecule from the organism's own molecules. Biochemically, the foreign antigen may be similar to autogenous antigens. Subsequently, a.