Genetic control of major complex histocompatibility antigens.

The histocompatibility loci of the MHC can be separated into two functionally distinct types. One, loci first defined by lymphocyte reactivity in MLC, LD loci, the phenotypic expression of which leads to proliferation of allogeneic T cells, and two, serologically defined, SD loci, products of which act as targets for cytotoxic lymphocytes. Although the SD loci may be definable by both serological techniques and by lymphocyte reactions in CML, and it may well be that the LD loci products will be defined serologically, the functional difference between them is documented by the apparently converse roles in MLC and CML. The LD loci are most effective in leading to MLC stimulation and do not function as targets in CML for reasons discussed elsewhere; the SD loci function poorly it at all in stimulating proliferation in MLC but are excellent targets in CML. A cellular dichotomy may exist in reaction to these different genetic components of the MHC.