Psoriasis is a common autoimmune skin condition that doctors believe is caused by skin cells reproducing too quickly, therefore the defense system turns on the body and attacks the person's skin. It then creates thick, crimson colored patches or plaques overlying white, silvery scales. These can cause discomfort; mostly itchiness and soreness. These patches can be small, but in more severe cases can be quite large. Most commonly these patches appear on the knees, elbows and scalp, though they can affect any part of the skin.