Formation of C3-IgG complexes in serum by aggregated IgG and by non-immunoglobulin activators of complement

We studied the generation of C3-IgG complexes during the activation of C3 in serum by aggregated human IgG (AHG), zymosan or cobra venom factor (CVF). C3-IgG complexes were detected by specific radioimmunoassays: samples to be tested were incubated with anti-IgG Sepharose, and complexes that had bound to the Sepharose were detected by incubation with either ¹²⁵I-anti-C3c or ¹²⁵I-anti-C3d,g. Incubation of serum with as little as 6 μg AHG per ml, for 30 min at 37°, resulted in the generation of C3-IgG complexes. When serum was incubated with zymosan or CVF, C3-IgG complexes were also generated. AHG appeared to be more effective in the generation of C3-IgG complexes than CVF. We calculated that AHG (2 mg/ml) caused about 36% of the C3 to be fixed to IgG, CVF (400 μg/ml) about 14%. Finally, the presence of C3 fixed to IgG in serum incubated with CVF was demonstrated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE), followed by immunoblotting. This study indicates that the formation of C3-IgG complexes in serum is not only induced by immune complexes but also by non-immunoglobulin activators. Therefore, C3-IgG complexes might be considered as complement activation products, and their detection in patients' samples should not be considered as conclusive evidence for the presence of immune complexes.