IgM rheumatoid factors capable of complement fixation and activation are commonly present in the sera of adults with rheumatoid arthritis. Hidden complement fixing IgM rheumatoid factor has been demonstrated in the majority of patients with juvenile RA and hidden agglutinating IgM rheumatoid factors have been demonstrated in the serum of adults with seronegative rheumatoid arthritis. We studied...

A sensitive radioimmunoassay has been applied to the study of rheumatoid factor activity of IgM and IgG subclasses in serum and synovial fluid of patients with rheumatoid arthritis. Although rheumatoid factor activity was found in all four subclasses, its distribution was not the same as that of the total subclasses in normal sera. Results are discussed in relation to the possible clinical valu...