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Serum IgG and IgM rheumatoid factors and complement activation in extraarticular rheumatoid disease.

Rheumatoid factors (RF) may participate in both synovial and extraarticular (EA) inflammation in rheumatoid disease (RD). The relative roles of serum IgG RF and IgM RF in extraarticular rheumatoid disease (EARD) are unclear, as is the importance of complement (C) activation by these proteins. To investigate the relation of C activating properties of IgM RF (RF CAP) and total IgG RF and IgM RF to EARD we compared 18 patients with only articular disease to 27 patients with various EA manifestations (nodules, cutaneous vasculitis, neuropathy, Felty's syndrome) using radioimmunoassays for IgG and IgM RF and an established hemolytic assay for C activation by IgM RF. We calculated RF CAP by determining the mean hemolysis of sensitized SRBC/ml of RF serum (MH/ml). Normal volunteers and patients with other inflammatory arthritides served as controls. Controls had negligible amounts of IgG RF, IgM RF, and RF CAP. Mean IgG and IgM RF levels and RF CAP values were significantly higher in patients with EARD than those in the arthritis-only (AO) group. Mean IgM RF concentrations and IgM RF CAP correlated with each other and EARD. IgG RF also correlated with IgM RF and EARD, but did not contribute to RF CAP or EARD when adjusted for IgM RF. Further, some patients had high RF CAP values despite modest IgM RF levels. These data suggest that quantitative differences in IgM RF CAP and total IgM RF may be more important than IgG RF as determinants of EARD.