IL17A and psoriatic arthritis: Interleukin 17A (IL-17A) represents a pleiotropic cytokine that has gained attention as signature cytokine of CD4+ T helper 17 (Th17) cells and has been put forward as critical determinant of psoriasis, a chronic relapsing T cell-mediated inflammatory disorder of the skin, and rheumatic musculoskeletal diseases like psoriatic arthritis and peripheral and axial spondylarthritis.