The requirement of more than one antigenic determinant for immunogenicity.

Rabbits primarily stimulated with a BSA (bovine serum albumin)-sulfanilic acid complex will produce a good secondary response to the sulfanilic acid hapten if the carrier used in the secondary stimulus is again BSA, and not if the secondary carrier is HGG (human gamma globulin). In the latter situation, a good secondary response is obtained, however, if the rabbits are pretreated a few weeks earlier with free HGG. We conclude that the immune stimulus involves the recognition of carrier determinants unrelated to the hapten. As the receptors for recognition of unrelated determinants are probably situated on different cells, we suggest that the immune stimulus leading to antibody formation requires the interaction of two antigen-bridged cells.