Psoriasis is a skin disease that causes scaling and inflammation (pain, swelling, heat, and redness). Skin cells grow deep in the skin and slowly rise to the surface. This process is called cell turnover, and it takes about a month. With psoriasis, it can happen in just a few days because the cells rise too fast and pile up on the surface. Most psoriasis causes patches of thick, red skin with silvery scales. These patches can itch or feel sore. They are often found on the elbows, knees, other parts of the legs, scalp, lower back, face, palms, and soles of the feet. But they can show up other places such as fingernails, toenails, genitals, and inside the mouth.

Psoriasis begins in the immune system, mainly with a type of white blood cell called a T cell. T cells help protect the body against infection and disease. With psoriasis, T cells are put into action by mistake. They become so active that they set off other immune responses. This leads to swelling and fast turnover of skin cells. People with psoriasis may notice that sometimes the skin gets better and sometimes it gets worse.