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Rheumatoid arthritis (RA) patients often develop rheumatoid factors (RFs), antibodies that bind IgG Fc, and anti-modified protein antibodies (AMPAs), multireactive autoantibodies that commonly bind citrullinated, homocitrullinated, and acetylated antigens. Recently, antibodies that bind citrulline-containing IgG epitopes were discovered in RA, suggesting that additional undiscovered IgG epitopes could exist and that IgG could be a shared antigen for RFs and AMPAs. This study was undertaken to reveal new IgG epitopes in rheumatic disease and to determine if multireactive AMPAs bind IgG.Using sera from patients with RA, systemic lupus erythematosus, Sjögren's disease (SjD), or spondyloarthropathy, IgG binding to native, citrulline-containing, and homocitrulline-containing linear epitopes of the IgG constant region was evaluated by peptide array, with highly bound epitopes further evaluated by enzyme-linked immunosorbent assay (ELISA). Binding of monoclonal AMPAs to IgG-derived peptides and IgG Fc was also evaluated by ELISA.Seropositive RA sera showed high IgG binding to multiple citrulline- and homocitrulline-containing IgG-derived peptides, whereas anti-SSA+ sera from SjD patients showed consistent binding to a single linear native epitope of IgG in the hinge region. Monoclonal AMPAs bound citrulline- and homocitrulline-containing IgG peptides and modified IgG Fc.The repertoire of epitopes bound by AMPAs includes modified IgG epitopes, positioning IgG as a common antigen that connects the otherwise divergent reactivities of RFs and AMPAs.