Psoriasis is a disorder/disease caused by an overactive immune system response. With psoriasis, skin cells mature quicker than normal but shed at a normal rate. This causes a build up of dead skin cells that form lesions. The lesions are usually red and crusty. Symptoms include itchiness, red spots, scales and blisters. Psoriasis can be “set off” by stress, medications, infection or weather changes, for example.