Cryoglobulins isolated from sera of patients with rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) were analysed for their immunoglobulin, antibody, and complement components. In both disease categories the cryoglobulins contained predominantly IgG with lesser amounts of IgM and IgA, but relative to serum more IgM was concentrated in the cryoglobulins. IgM rheumatoid factor was found in 65% of RA cryoglobulins but in only 17% of SLE cryoglobulins (p less than 0.02), whereas SLE cryoglobulins contained more DNA binding activity than RA cryoglobulins (p less than 0.01). C1q binding activity was detectable in the majority of SLE and RA sera and SLE cryoglobulins. Paradoxically only two out of 34 RA cryoglobulins bound C1q, although rheumatoid factor activity was present in both cryoglobulins and sera. When isolated from serum the rheumatoid factor fraction strongly bound C1q. Both RA and SLE cryoglobulins contained similar small amounts of C3 and C4. Differences in antibody composition and complement binding activity of cryoglobulins from RA and SLE sera may reflect properties of immune complexes which affect their tissue localisation and pathogenicity.
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