Low molecular weight IgM in rheumatoid arthritis and other rheumatic diseases.

Low molecular weight (LMW) IgM was measured in the serum and synovial fluid of patients with rheumatoid arthritis (RA) and other rheumatic diseases. High levels were seen in RA, particularly in rheumatoid vasculitis and Felty's syndrome, and significant correlations occurred between LMW IgM and the rheumatoid factor (RF) level and other indices that reflected active or severe disease. LMW IgM-RF, measured by radioimmunoassay in those column fractions containing LMW IgM, correlated significantly with LMW IgM (P less than 0.005); preliminary experiments suggested that in some sera, a considerable proportion of the LMW IgM consisted of LMW IgM-RF. We conclude that LMW IgM and LMW IgM-RF may have important implications in the immunopathogenesis of RA and other rheumatic diseases.