•Synovial fluid anti-MAA (IgG, IgM) higher in RA vs. OA•Anti-MAA (IgG, IgM, IgA) higher in paired synovial fluid vs. serum in RA•Marked co-localization of MAA within ectopic lymphoid tissue in RA synovium with citrulline and CD19+/CD27+ B cells (r >0.75). The gene discussed is CD79A; the disease is rheumatoid arthritis.