Psoriasis is a noncontagious, chronic skin disease that produces plaques of thickened, scaly skin. The dry flakes of silvery-white skin scales result from the excessively rapid proliferation of skin cells. Psoriasis is fundamentally an immune system problem. The proliferation of skin cells is triggered by inflammatory chemicals produced by specialized white blood cells called T-cells. Psoriasis commonly affects the skin of the elbows, knees, and scalp.