IgM and IgG anti-F(ab')2 antibodies in rheumatoid arthritis and systemic lupus erythematosus.

Radioimmunoassays for anti-F(ab')2 antibodies, which feature the use of goat anti-human Fc antibody for correcting potentiation of IgM anti-F(ab')2 antibody titers by endogenous IgM anti-Fc antibodies (rheumatoid factors), are described. Individuals with classic rheumatoid arthritis had significantly more IgM anti-F(ab')2 antibody (P less than 0.001) and IgG anti-F(ab')2 antibody (P = 0.05) than did individuals with systemic lupus erythematosus or normal volunteers. There is some similarity in patterns of isotype distribution of anti-F(ab')2 antibodies and rheumatoid factors.