Research into the immunopathogenesis of psoriasis has resulted in several highly specifi c therapies that target components of the immune system. To understand how these therapies act against psoriasis, information is needed about the complex pathophysiology of the disease (fi gure 3). We focus here on the central mechanisms of the disease: cross-talk between innate and adaptive immunity and the central role of TNFα; the interleukin-23/T helper cell 17 (Th17) axis; and the eff ect of immune reactions on other cells in the skin