Prevalence of IgM, IgA and IgG rheumatoid factors in juvenile rheumatoid arthritis.

Using an enzyme immunoassay, sera from 50 children with juvenile rheumatoid arthritis (JRA) and 39 controls were tested for IgM, IgA and IgG rheumatoid factors (RF). RF of the IgM and IgA isotypes were present in 11 (22%) patients, but in only one control (p = 0.008). IgG RF was present in the sera of 2 (4%) patients and in none of the controls (p = 0.21). Of the 22 patients with IgM RF or IgA RF, only 3 sera (14%) contained RF of both isotypes. IgM RF was more common in patients with polyarticular disease, while IgA RF was more common in patients with pauciarticular disease. These results indicate that IgM and IgA RF are present in a significant minority of JRA patients and suggest that there is independent expression of the respective RF isotypes.