As published elsewhere (Ferreira et al., J. Exp. Med., in press) mouse serum contains a 10S β-globulin which specifically binds to fluid phase activated Ss (C4) protein. This new protein was named Ss-bp or C4-bp. A search was made in human serum for the presence of a protein with similar properties. Human serum was incubated at 37°C with washed immune precipitates and centrifuged. Activated C4 was isolated from the supernatant by precipitation with a monospecific antiserum to C4, and injected into rabbits. The resulting antiserum was tested against whole human serum by crossed-immunoelectrophoresis. It reacted only against C4 and an additional serum component with properties very similar to those of mouse C4-bp.
Human C4-bp is a heat stable β-globulin which is found in the exclusion volume after filtration of serum or EDTA-plasma through a Sephadex G-200 column. It has a sedimentation coefficient of 13S as determined by sucrose gradient ultracentrifugation of serum from a patient with hereditary angioedema (HAE) with low levels of C4.