IGHG1 and rheumatoid arthritis: Interestingly, a very recent study identified that human synovial lymphocytes exhibit enhanced class-switching to IGHG3 and IGHG1 (the most common IgG isotype in humans (48), similar in abundance to Ighg2b in mice) with corresponding systemic manifestations associated with the degree of synovial lymphocytic infiltration in RA (49).