Pathogenesis: Psoriasis is an autoimmune disease caused by an abnormality within the cells of the immune system. There is a genetic susceptibility, and the human leukocyte antigen (HLA) Cw6 locus is the most commonly found (but not the only) susceptibility factor in patients who develop psoriasis. The success of therapy with cyclosporine, a medication that dramatically decreases T-cell function, was one of the first clues to the pathogenesis of psoriasis. Psoriatic patients given this medicine almost always have rapid clinical improvement.