The specificity of T lymphocyte responses to chemically defined antigens.

A system is described that allows the definition of T cell receptor specificity with some precision. It involves immunization of guinea pigs with hapten coupled to mycobacteria. The T cells of such animals respond to many but not all carriers modified by that hapten. Such T cells recognize neither hapten nor carrier alone, but rather determinants involving both the hapten and the carrier. No evidence for hapten-specific T cells was found. A model of the antigen binding site of the T cell receptor emerged from these experiments. According to this model, the T cell receptor consists of a single site of relatively large extent involving multiple subsites which are of low and roughly equal affinity. Thus, the haptenic group is not immunodominant for T cells as it is for B cells and for anti-hapten antibody. This suggests that the antigen binding receptor on T cells differs in some fundamental way from that on B cells. It is proposed that antigen recognition by T cells is mediated by an immunoglobulin heavy chain variable region that is not paired with an immunoglobulin light chain variable region.