What is the connection between Psoriasis and auto immune?

From eHealthMD:

Evidence from recent research studies strongly suggests that psoriasis is a disorder of the immune system. Normally, the immune system defends the body from infection by bacteria, viruses, and other invaders. Sometimes, however, the immune system makes a mistake and attacks the cells, tissues, and organs of a person's own body. When this happens, the resulting disease is called an autoimmune disease. Many autoimmune diseases run in families. The theory that psoriasis is an autoimmune disease is based on the fact that scientists have found abnormally large numbers of T cells (a type of white blood cell) in the red, flaky skin patches of people with psoriasis. T cells are the infantry of the immune system. When the body senses a need to defend itself against infection, it first makes and then releases millions of T cells to fight off the invaders. Some t cells are normally found in skin. The presence of abnormally large numbers of T cells in skin affected by psoriasis suggests that the immune system is attacking the skin by mistake. (http://www.ehealthmd.com/library/psoriasis/pso_causes.html)