IL17A and rheumatoid arthritis: One subset of T cells, T helper (TH)1 cells, produce IFNγ and another subset, TH2 cells, produce IL-4 [4]; but as IFNγ and IL-4 are found at very low levels in the RA joint [5], the source of IL-17 in the RA joint was unclear until the recent discovery of a third subset of T cells capable of producing IL-17, the TH-17 cells.