C3 and Arthritis: We compared the proportions of all immune cell populations between patients with or without specific clinical (mucocutaneous involvements, arthritis, nephritis and hematological involvement), serological features (anti-nuclear antibody titer, anti-dsDNA antibody titer, anti-SSA antibody, anti-SSB antibody, anti-RNP antibody, anti-Smith antibody, serum C3 level, serum C4 level, IgG/IgM/IgA/IgE level) or inflammatory markers (erythrocyte sedimentation rate and CRP level).