A solid-phase radioimmunoassay for IgG and IgM antigammaglobulin factor in rheumatoid arthritis sera

Rheumatoid factors of IgG and IgM class were detected by means of a solid-phase radioimmunoassay using formalinized sheep erythrocytes sensitized with rabbit anti-sheep erythrocyte serum and 125I-labelled anti-human IgG or anti-human IgM antibody. 28 of 35 sera and all of 7 synovial fluids of patients with rheumatoid arthritis showed positive reaction for both IgG and IgM rheumatoid factor, though the activity of IgM factor was mostly higher than that of IgG factor. However, rheumatoid factor present in synovial fluids of 3 seronegative patients was mainly of IgG class. Of 14 sera of patient with other collagen diseases, 11 showed IgG and 6 showed IgM rheumatoid factor. The IgG rheumatoid factor seemed to be associated with autologous IgG to form macro-molecular complexes which could be dissociated into 7S molecules of IgG rheumatoid factor and IgG by acid treatment.