There are four types of response to an antigen: the production of classical antibody; the induction of hay fever type of hypersensitivity; the induction of tuberculin type of hypersensitivity; and the induction of specific immunological tolerance. Since the formation of antibody of some kind appears to be associated with the first three types of response, and the inhibition of antibody formation with immunological tolerance, any hypothesis of antibody formation must be consistent with what is known about all four responses. A reasonable consistency may be achieved by postulating the existence in the cell of endogenously produced 'genocopies' of certain proteins, which act as templates and determine the configuration of the proteins as they are synthesized; when a genocopy is modified by contact with antigen, protein will be synthesized with the specific complementary pattern of an antibody. The interference with this synthesis during embryonic life by entry into the cell of macromolecules objectionable to it, results in intracellular antibody and self-replicating mechanism for its further production—a system that can deal with all such macromolecules at the intracellular level; this corresponds to specific immune tolerance. When antigenic material enters body cells after birth, the same response occurs but there is in addition a transfer of the self-replicating mechanisms to cells of the lymphoid system with a consequent output of extracellular antibody—the classical antibody response. The hypersensitivity responses are intermediate in kind, and differ according to the transferability of the self-replicating mechanism, the amount of antibody released into the body fluids, and the readiness with which the antibody becomes attached to the tissue cells.