These patches, which are sometimes referred to as plaques, usually itch and may burn. The skin at the joints may crack. The disease may also affect the fingernails, the toenails, and the soft tissues inside the mouth and genitalia. About 15 percent of people with psoriasis have joint inflammation that produces arthritis symptoms. This condition is called psoriatic arthritis.
Recent research indicates that psoriasis is likely a disorder of the immune system.
This system includes a type of white blood cell, called a T cell, that normally helps protect the body against infection and disease. Scientists now think that, in psoriasis, an abnormal immune system causes activity by T cells in the skin. These T cells trigger the inflammation and excessive skin cell reproduction seen in people with psoriasis. In about one-third of the cases, psoriasis is inherited. Researchers are studying large families affected by psoriasis to identify a gene or genes that cause the disease.