Role of rheumatoid factor in complement fixation and indirect hemagglutination tests for immunoglobulin M antibody to cytomegalovirus.

Absorption of immunoglobulin M (IgM)-rheumatoid factor (RF) from serum samples by reaction with insolubilized gamma globulin reduced the complement-fixing (CF) antibody titer to cytomegalovirus (CMV) antigen to less than 1:2 in the IgM fraction of some, but not all, sera. Thus, IgM-CF activity in some sera appeared to be due to specific IgM anti-CMV antibody and in other sera to complexes of IgM-RF with antiviral IgG antibody. Prozones were present in the CF tests on IgM fractions. Increasing the concentration of antigen from 2 to 4 U reduced the prozone titer by one or two double dilutions. This observation suggested that a competition for antigen may be operating at low dilutions of IgM antibody fractions. Removal of RF had little or no effect on the reaction of the IgM fraction of sera with CMV by the indirect hemagglutination test.