Histamine release from synovium-derived, cultured MCs following FcγRI and FcεRI aggregation did not differ significantly between OA and RA patients (Fig. 2A,E), while PGD2 production by the RA patients’ MCs was significantly higher than by the OA patients’ MCs (Fig. 2B,F). Here, PTGDS is linked to rheumatoid arthritis.