Fc-mediated immune precipitation. IV. Antigen dependency and specificity.

The antigen dependency of Fc-mediated immune precipitation was investigated by comparing the immune precipitation of three different protein antigens (bovine serum albumin, BSA, human plasma transferrin, HPT, and human fibrinogen, HFg) using specific intact rabbit IgG antibodies and isomolar solutions of the corresponding F(ab')2 fragments. We found that this precipitin mechanism is highly antigen-dependent both quantitatively and qualitatively. Sucrose density gradient ultracentrifugation was used for the identification of Fc-precipitating immune complexes, and we demonstrated that different types of complexes take part in Fc-mediated immune precipitation in the different antigen-antibody systems. Thus, only small complexes are involved in the HFg system, whereas small as well as large complexes precipitate by this mechanism in the BSA and the HPT systems. The specificity of Fc-mediated immune precipitation was studied by mixing human serum albumin (HSA)-anti-HSA IgG complexes with either HPT-anti-HPT IgG or HFg-anti-HFg IgG complexes. These investigations show that Fc-mediated immune precipitation is a specific process due to the preferential binding of homologous complexes.