Kinetic and static analyses of complement fixation (CF) reactions were carried out using EA, EA (human), solutions of human thyroglobulin-rabbit anti-thyroglobulin and latex particles coated with human γ-globulin (LPF II). Heated rheumatoid arthritis (RA) sera and N sera, rheumatoid factor (RF) and inhibitor (Ag-Ab)—a γ1M-globulin isolated from RA sera—were tested for their effect on CF reactions.Addition of RA serum and N serum to EA and gpC′ delayed immune hemolysis. Preabsorption of the sera with solid heat denatured γ-globulin did not affect their inhibitory activity.Inhibitor (Ag-Ab) was shown to delay lysis of EA by gpC′. The substance also inhibited CF by soluble antigen-antibody complexes, but CF reactions involving LPF II remained unaffected.RF did not interfere with the lysis of EA by gpC′. This substance was also inactive in CF reactions involving soluble antigen-antibody complexes. RF, however, inhibited CF by LPF II, especially when preincubated with LPF II for 90 min at 56°C.There was also a decrease in the latex fixation titer of RA sera when LPF II was preincubated with gp serum, suggesting some competition between RF and gpC′ for sites of attachment on the γ-globulin molecule.
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