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The significance of IgM antinuclear antinuclear antibody in rheumatoid arthritis and other connective tissue diseases.

An investigation of the incidence of IgG and IgM antinuclear antibodies (ANA) in patients with connective tissue diseases showed that IgM ANA predominated in rheumatoid arthritis, whilst in systemic lupus erythematosus IgG antibodies were more common. Patients with other connective tissue diseases less frequently had antinuclear antibodies and there was little difference in the incidence of IgG and IgM antibodies. Cross reacting rheumatoid factors contributed to the IgM antinuclear activity of rheumatoid sera. IgM ANA was not related to disease activity in patients with rheumatoid arthritis. Immunoglobulin class differences of ANA in rheumatoid arthritis and systemic lupus erythematosus may reflect the differing pathological processes of these two diseases and may be of value in their differential diagnosis.