The activity of the “C3b Inactivator Accelerator,” described at the Sixth Complement Workshop, has since been found to reside in β1H globulin, a 150,000 m.w. glycoprotein (K. Whaley and S. Ruddy, Science 193:1011–1013, 1976; and J. Exp. Med. 144:1147–1163, 1976). This protein accelerates the rate of inactivation of the hemolytic activity of the C3b by C3b Inactivator and also increases the rate of proteolysis of C3b by C3b Inactivator. In addition, β1H has direct effects on C3b, preventing its participation in either the classical or alternative pathway C5 convertases, and increasing the rate of decay of factor B from the alternative pathway convertase, .
Some of these apparent diverse effects of β1H may be explained by its binding to C3b. This is evidenced by 1) agglutination of cellular intermediates exposed to β1H by monospecific antibody to β1H, 2) immunofluorescent staining of such intermediates with fluorescein-labeled anti-β1H, 3) depletion of β1H activity from the fluid phase of reactions containing C3b.