It is generally accepted that T helper cells, include Th1, Th17 and Th22 cells, contribute to the development of psoriasis by secreting various cytokines including interferon γ (IFN-γ), tumor necrosis factor α (TNF-α), interleukin 17 (IL-17), and IL-223, 4, which results in the excessive proliferation and aberrant differentiation of keratinocytes. Here, IL17A is linked to psoriasis.