IL17A and psoriasis: This suggests a role for T helper cells type 1 (Th1 cells, characterized by production of interferon γ and tumor necrosis factor α) and/or T helper cells type 17 (Th17 cells, characterized by secretion of IL-17A/F and IL-22) in the pathogenesis of psoriasis as the differentiation/stability of these cells are stimulated by IL-12 and IL-23, respectively.