IL17A and psoriasis: The level of IL-9 has been found to be increased in both the serum and the lesions of patients with psoriasis and positively correlated with the severity of psoriasis and impaired quality of life (QoL).157 Cells producing IL-9 (the Th9 cells) are also found increased in the skin lesions of psoriasis.158 IL-9 can promote the secretion of IL-17, IL-13, IFN-γ, and TNF-α in psoriasis.158 However, the mechanisms of how IL-9 regulates the microenvironment of psoriasis still need further research.