IL17A and psoriasis: The pathogenesis of psoriasis lesions is attributed to the dysfunction of T-cell subsets including T-helper (Th) 1 cells, Th2, Th17, Th22 and regulatory T cells (Tregs) and the resulting aberrant release of the corresponding cytokines including IFN-γ, tumour necrosis factor (TNF)-α, IL-23 and IL-17 family members [25,26].