Psoriasis is an immune-mediated disease, that causes inflammation in the body. There may be visible signs of inflammation such as raised plaques and scales on the skin. This occurs because the overactive immune system speeds up skin cell growth. Normal skin cells completely grow and shed in a month. However in patients with psoriasis, skin cells do this in only three or four days. Instead of shedding, the skin cells pile up on the surface of the skin. Some people report that psoriasis plaques itch, burn, and sting. Plaques and scales may appear on any part of the body, although they are commonly found on the elbows, knees, and scalp.