Psoriasis is a T cell-mediated inflammatory disease in which  Th17 and Th22 T-cell subsets play a major role. Th-17 lymphocytes are highly activated in psoriatic skin, releasing inflammatory cytokines involved in psoriasis plaques. Additionally, Th-22 secreted cytokines activate the proliferation and differentiation of keratinocytes, resulting in the specific hyperplasia of psoriatic epidermis.