The adherence of soluble immune complexes to stimulated alveolar macrophages was studied in vitro using HSA-anti-HSA complexes prepared in antigen excess. Those complexes containing more than two molecules of antibody preferentially adhered to macrophages in the absence of complement. Free γG in less than physiological concentrations inhibited the adherence of complexes, and the presence of complement did not significantly alter this inhibition. Complexes prepared with reduced and alkylated antibodies showed a decreased adherence. The strength of binding of soluble complexes to macrophages and their efficiency in fixing complement were greater than seen with small aggregates of homologous γG. These differences in biological properties were observed even though the immune complexes and aggregates contained comparable numbers of γG molecules. The γG receptor on rabbit macrophages exhibited species specificity. Pretreatment of macrophages with proteolytic enzymes led to adherence of larger amounts of soluble complexes.
These observations suggest that the strength of binding of soluble immune complexes to macrophages and their efficiency in fixing complement are not determined solely by a random summation of individual binding sites. It is proposed that conformational changes in the γG antibodies or a specific molecular arrangement in the lattice work of complexes containing large protein antigens may influence the biological properties of the soluble complexes.