C4A and immunoglobulin G4-related sclerosing disease: Among 136 patients with the rheumatoid arthritis, the increased level of IgG4 in the serum was observed in 46% of patients and was correlated with the active disease [11].The elevated serum IgG4 levels were the most important laboratory features of IgG4-RD both in our and other publications [12, 13].The abnormal C3 and C4 complement levels were not observed in the presented cases, except the case with vasculitis, and they are not typical for IgG4-RD, but more frequently appear in other autoimmune disorders.