F antigen. III. Immunological and biological properties.

Relationship of the isoforms of F antigens from A2G and CBA inbred mouse strains have been worked out in detail in respect to their immunogenicity. Mice from several inbred strains have been shown to possess the ability of forming anti-F upon stimulation with xenogeneic F antigens from certain species. An immunoelectrophoretic method for sensitive quantitation of the antigen is described. During human ontogenesis the antigen appears very early. It could never be detected in serum except in cases of acute liver disease. A small fragment set free under denaturing conditions showed the ability of inhibiting precipitation of the complete antigen by antibody. The F protein seems to be a vehicle for this small fragment possessing haptenic properties and probably being responsible for the universal cross-reactivity.