IL17A and rheumatoid arthritis: Indeed, IL-17A is highly expressed in the central nervous system (CNS) lesions and in the blood and cerebrospinal fluid (CSF) of patients with MS [20–24], in the colonic mucosa of patients with ulcerative colitis or Crohn's disease [25, 26], in the psoriatic skin [27, 28], and in the synovial tissues from rheumatoid arthritis patients [29].