Human Complement C3b. In immunoelectrophoresis and radial immunodiffusion, using various antiserum concentrations against human plasma a single precipitin line is obtained and shows a reaction of full identity with the purified immunogen. The antiserum does not react with any other protein component of human serum or plasma. Cross-reactivity Inter-species cross-reactivity is a normal feature of antibodies to plasma proteins, since homologous proteins of different species frequently share antigenic determinants. Cross-reactivity of this antiserum has not been tested in detail.