The antibody titer of IgA anti-C1-INH367–385 Ac in patients with SLE was significantly lower than that in HCs by 1.36-fold (p = 0.0004), and that in patients with RA was significantly higher than that in patients with SLE by 1.31-fold (p = 0.0012). This evidence concerns the gene CD79A and rheumatoid arthritis.