Conversion of Soluble Immune Complexes into Complement-Fixing Aggregates by IgM-Rheumatoid Factor

Complement fixation was studied after aggregation of soluble immune complexes by IgM-rheumatoid factors (RF). Such aggregates fixed larger amounts of complement than did soluble complexes alone. The extent of this reaction was dependent upon the antigen-antibody ratio in the complex. Complement-fixing ability of those complexes formed near equivalence could be further enhanced by IgM-RF, while those formed in greater antigenic excess with little or no ability to fix complement could be aggregated and converted into complement-fixing units by IgM-RF. The composition of complexes that most readily reacted with RF to form aggregates contained two or more molecules of IgG antibody. Within the aggregate, the major share of complement fixation was mediated by the RF component rather than the IgG antibody. These observations are consistent with the view that RF blocks the binding site on IgG for complement and substitutes its own instead. Under the conditions employed, this substitution resulted in a net increase in complement fixation.