The immune system is the body's defense 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.
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.
WebMD Medical Reference from Healthwise