When you think of arthritis, you think of inflammation. Inflammation is a process in which the body's white blood cells and chemicals help protect us from infection and foreign substances such as bacteria and viruses.
In some diseases, however, the body's defense system (immune system) triggers an inflammatory response when there are no foreign substances to fight off. In these diseases, called autoimmune diseases, the body's normally protective immune system causes damage to its own tissues. The body responds as if normal tissues are infected or somehow abnormal.