IL17A and psoriatic arthritis: IL-17-producing mast cells are increased in the synovial fluid of patients with psoriatic arthritis [82] and NKp44+ ILC3 cells are also increased in the peripheral blood and skin in psoriasis [83] and produce IL-17 [84], indicating the IL-17-rich milieu in the inflamed skin and joints of PsA resulting from multiple innate and adaptive IL-17-producing cells.