A new complement function: solubilization of antigen-antibody aggregates.

Antigen-antibody aggregates are solubilized when incubated with fresh serum at 37 degrees, yielding immune-complexes of relatively small molecular weight which contain antigen, antibody, and complement (C3)determinants. Solubilization is complement-dependent,requires free Mg++ but not Ca++, and proceeds in sera from C4- or C5-deficient animals. It is accelerated in the presence of Ca++ in normal or C4-deficient guinea pig serum, suggesting involvement of the Cl-bypass activation of the properdin system. Immune precipitates can also be solubilized by monovalent fragments (Fab) of antibodies directed against determinants of the antibody molecules included in the antigen-antibody lattice. Similarly, it is suggested that complement-mediated solubilization might be induced by the combination of a complement fragment with the antibody in the immune-aggregate.