IL17A and psoriasis: Th17 cells have been demonstrated to play important roles in several autoimmune diseases and chronic respiratory diseases, such as rheumatic arthritis, psoriasis, multiple sclerosis, asthma, and sarcoidosis (18, 40, 41).Th17 cells are characterized by the hallmark cytokine of IL-17A, and have a critically important role in the induction of multiple autoimmune disorders (40).