The Presence of Anti‐Protein A Antibodies in Patients with Rheumatoid Arthritis

Sixteen patients with rheumatoid arthritis (RA) were examined for the presence of anti‐protein A antibodies. The F(ab′)₂ preparations from five RA patients showed significant binding lo IgG‐free protein A on ELISA. The protein A binding was further examined by immunoblotting. The F(ab′)₂ preparations of high protein A‐binding protein gave A specific reaction with IgG‐free protein A on nitrocellulose paper. This demonstrates the presence of anti‐protein A antibodies in patients with RA. Those RA patients with anti‐protein A antibodies had more active disease as judged by the Lansbury's activity index. The level of serum rheumatoid factor (RAHA) was significantly higher in patients with anti‐protein A antibodies than in those without anti‐protein A antibodies.