Psoriasis. Psoriasis is an autoimmune condition. Plaque psoriasis is the most common form of psoriasis, and shows up as areas of thick, red skin with white scales. Plaque psoriasis can be on any part of the body, often in the elbows, knees, scalp, back and face. Another form of psoriasis is called psoriatic arthritis, which can can cause joint ....