Rat major histocompatibility RT1.C antigens of restricted tissue distribution consist of two polypeptide chains with molecular weights of about 42 000 and 12 500.

The RT1.C region has recently been defined genetically as a third region of the rat major histocompatibility system, RT1, and has been shown to code for cell-surface antigens with restricted tissue distribution (present on lymphocytes and absent from red blood cells and liver) and for target antigens of unrestricted cytotoxic T lymphocytes. Immunoprecipitation with C-antigen-specific alloantisera and SDS-PAGE analysis, reveal that C-region products are glycoproteins composed of molecules of 40 500-43 000 and 12 500 molecular weight, respectively. Thus, the RT1.B region, which codes for class II molecules, is flanked by RT1 regions which determine class I molecules (as defined biochemically), which are either expressed ubiquitously (RT1.A antigens) or in a restricted manner (RT1.C products). The homology to H-2Qa antigens is discussed.