IL17A and autoimmune disease: TH17 effector cells secrete at least one of the six cytokines belonging to the IL-17 family, that is, IL-17A, IL-17B, IL-17C, IL-17D, IL-25 and/or IL-17F; however, IL-17A, the signature cytokine, has received the greatest attention in studies of autoimmune diseases [15].