Immunochemical characterization of murine H-2 controlled Ss (serum substance) protein through identification of its human homologue as the fourth component of complement

The S region of the mouse H-2 complex is genetically defined by a series of alleles, or pseudoalleles, which control the level of a serum globulin, Ss, and of its allotypic vairatn, Slp. In contrast with the products of other genes in the complex, no homologue of the Ss protein has been found in other species, except the rat. In the present study, a component in human plasma was identified which cross-reacts with anti-mouse Ss, and which also displays electrophoretic and size similarities to the mouse Ss protein. This component was isolated and characterized immunochemically as the fourth component of human complement (C'4 or beta1E globulin). Several antisera monospecific for human C'4 cross-react with the mouse Ss protein and detect its genetically determined quantitative variations.