What is an autoimmune disease?

Mihaela B. Taylor, MD
An autoimmune disease is any condition in which the body's immune system attacks itself. This happens when the immune system, which normally protects the body from foreign invaders such as bacteria and viruses, starts attacking substances and tissues normally present in the body. Examples of autoimmune diseases are lupus and rheumatoid arthritis.
The immune system is responsible for protecting us from disease. It does so by identifying and destroying foreign bodies, such as infectious agents and unwanted bacteria. However, the immune system is not always able to differentiate between "good" and "bad" cells. Autoimmune diseases occur when the immune system mistakes the body's tissues for foreign bodies and attacks the body's own cells. Abnormal antibodies in the bloodstream are usually responsible for these cases of "mistaken identity."
Anthony L. Komaroff, MD
Internal Medicine
Occasionally, for reasons only partially understood, the immune system mistakenly turns against the very body it's designed to protect, resulting in conditions known as autoimmune diseases. Experts believe as many as 50 million Americans, or one in five people, may be afflicted with an autoimmune disease, and some may have more than one. Because women, particularly premenopausal women, are affected about three times more often than men, scientists suspect hormones are involved, but how is not clear.

Genes also play a role, since autoimmune diseases run in families. However, different family members can have different autoimmune diseases. But genetic predisposition is not the only factor in determining who develops disease. Infection with bacteria and viruses, exposure to toxins or certain drugs, long-term stress, aging, and pregnancy may also influence an individual's susceptibility to developing an autoimmune disease. And more important than previously realized, the microbes that colonize the human body right from birth influence the development of the immune system and may play a role in whether you develop an autoimmune disease.

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