IL21 and psoriasis: Previous reports suggest that STAT3 plays a crucial role in Th17 and Treg cell differentiation and deregulation of STAT3 leads to the psoriasis pathogenesis through mediation of IL-17/IL-22/ IL-23 axis signaling; also the cytokines such as IL-6 and IL-21 are critical for Th17 cells maintenance that functions mainly through STAT3, also an impaired IL-17 production was observed in the absence of STAT3 [79, 80].