A disease of the immune system
While the exact cause of psoriatic arthritis has not been determined, it is believed that certain changes in the immune system are involved. The immune system normally protects your body against foreign invaders such as bacteria, fungi and viruses. But with psoriatic arthritis, the immune system loses its ability to tell the difference between these foreign invaders and the body's normal cells and begins to attack the body's cells, too. This causes inflammation in the lining and connective tissues of the joints, as well as scaling, redness, and inflammation on the skin.