IGHG3 and rheumatoid arthritis: In RA synovium Sato et al. have found a more complement-dependency of inflammation with IgM-RF and IgG3 than with other IgG subclasses suggested a dominant role of IgG3 in RA (31), both IgM and IgG3 are the most efficient of the human immunoglobulin (sub)classes in activating complement.