The immune system is the body's defence against foreign substances, such as bacteria or viruses, that may be harmful. An autoimmune disease is an abnormal condition that occurs when a person's immune system attacks its own tissues as though they were foreign substances.

• Normally, when a foreign substance enters the body, the immune system creates special cells to attack and destroy the foreign substance. These cells include antibodies and white blood cells (lymphocytes).
• In a person with an autoimmune disease, the immune system recognizes some of the person's own tissues as foreign substances. The body makes antibodies and other cells that attack and destroy these tissues. This process often leads to inflammation and eventually, if it continues, scarring and destruction of the organs that are made up of those tissues.

Why the body attacks its own cells is not known. Autoimmune diseases include lupus, rheumatoid arthritis, scleroderma, and Sjögren's syndrome. Certain types of diabetes and thyroid disease are related to autoimmune reactions. People who have autoimmune diseases are at an increased risk for infections.