The major rheumatoid factor crossreactive idiotype and IgA rheumatoid factor in juvenile rheumatoid arthritis.

One third of patients with juvenile rheumatoid arthritis (JRA) are seronegative for classic or hidden IgM rheumatoid factor (RF) yet express the major RF crossreactive idiotype (RCRI). We studied 60 children with JRA and 57 pediatric controls for IgA RF and RCRI expression to determine whether RCRI is associated with IgA RF in JRA. Twenty-one patients had IgA RF using rabbit or human IgG as substrate. Twenty-seven patients with JRA expressed high concentrations of RCRI in their sera. Only 9 of these had IgA RF. Eleven of the RCRI+ sera contained neither IgA RF nor classic or hidden IgM RF, and IgA RF was found in patients with all JRA onset subtypes. In JRA, RCRI may be expressed on either IgG RF or on parallel set antibodies without RF activity.