Sites and Mechanisms of Complement Interaction with Guinea Pig Immunoglobulins

Guinea pig γ1 immunoglobulins incubated as preformed aggregates with undiluted guinea pig serum initiate their attack on the C sequence at C3, apparently bypassing C1, C4, and C2. This C interaction site is located on the F(ab′)2 fragment as demonstrated by the full retention of activity after digestion with pepsin. γ2 Immunoglobulins seem to possess two C interaction sites, one on the Fc fragment and the other on the F(ab′)2 fragment. Pepsin digestion of the γ2 antibodies reduced their C-fixing activity and also changed their mode of C interaction. Whereas the intact molecule utilized all nine C components, the F(ab′)2 fragment no longer reacted with C2 but diminished the activity of late components. Both types of immunoglobulins utilized C more efficiently at 37°C than at 4°C when incubated as preformed aggregates. However the soluble γ2 complexes exhibited enhanced C consumption at 4°C.