IL17A and psoriasis: CD4+ T cells producing interleukin (IL)-17, named Th17 cells, play an essential role in its pathogenesis (129). Th17-derived cytokines, IL-17A, IL-17F and IL-22, induce epidermal acanthosis, which represents an intriguing histological finding of psoriasis and results from the proliferation of epidermal keratinocytes.