Structure of human IgM rheumatoid factor Fab in complex with its autoantigen IgG Fc. Molecular model of the structure of human IgM rheumatoid factor in complex with its autoantigen IgG Fc. Rheumatoid factors are the characteristic autoantibodies of rheumatoid arthritis, which bind to the Fc regions of IgG molecules. The structure implicates antigen-driven selection, following somatic mutation of germline genes, in the production of pathogenic rheumatoid factors.