IGHG3 and kidney disorder: Observations indicating that both IgG2a and IgG3 antibodies are contributory to kidney disease raise the possibility that IgG3 rheumatoid factors (i.e. antibodies able to bind to IgG Fc regions), which can exhibit specificity for IgG2a Fc regions and are present at high frequency in MRL/lpr mice [16,17], exacerbate the inflammatory stimulus created by the renal deposition of IgG2a antibodies.