Differential cellular immune responsiveness to systems of the major histocompatibility complex.

Antigens determined by genes of the major histocompatibility complex can be divided into two systems" the LD (lymphocyte-defined or L determinant) and SD (serologically defined or S determinant) antigens. This division is based on the differential cellular responsiveness to these two sets of antigens of two subpopulations of T lymphocytes: a proliferating helper cell responds primarily to the LD antigens while a cytotoxic T lymphocyte responds most strongly to the SD antigens.