Shared Idiotypic Determinants on B and T Lymphocytes Reactive Against the Same Antigenic Determinants

In the rat the major histocompatibility locus antigens are determined by the Ag‐B locus. In the present article evidence is presented that the sera and urine ui normal adult rats contain naturally occurring antibody‐like molecules with reactivity to allogeneic Ag‐B antigens. Such molecules can be shown to contain both antigen‐binding capacity for the relevant antigens and the idiotypic markers signifying such specific reactivity. The molecules could be shown to be composed of two groups of molecules, one around 7S IgG in size and the other around 35,000 in molecular weight. Only the smaller molecules were found in the urine. From other data we know that the 7S molecules are produced by B lymphocytes and the 35,000‐molecular‐wfight molecules by T cells. Purified natural anti‐Ag‐B factors, when inoculated into rabbits or chickens, lead to the production of specific anti‐idiotypic antibodies that will selectively inactivate rat T lymphocytes with the capacity to react against the relevant Ag‐B antigens while leaving other reactivity intact. We thus conclude that the present system allows the purification of naturally occurring idiotypic B‐ and T‐cell products with antigen‐binding specificity for further biochemical and functional analysis.