TNF and psoriasis: Systemic inflammation and consecutively exacerbated activation of keratinocytes promote the formation of psoriasis skin lesions by activating the immune cascade involving dendritic cells, T-helper lymphocytes (chiefly Th1, Th17, and Th22), and proinflammatory molecules (mainly IL-1, IL-6, TNF-α, IL-12, and IL-23).