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Q: I keep hearing the term "autoimmune disease." What does it mean?

A: In most people, the immune system does a good job of protecting them from infection. But sometimes the immune system mistakenly turns against the very body it's designed to protect. When this happens, the resulting conditions are known as autoimmune diseases.

At the earliest point in our lives, our immune system's cells are circulating around our bodies learning to recognize the tissues they'll be living with. The immunologists say that the immune system is learning to recognize "self." Only after it has learned what "self" is can it determine what is "non-self," or foreign.

When the immune system recognizes something as foreign, it attacks. The immune system is like an army: It has many different types of attackers, and it has a command structure. Certain cells order the attack to turn on and off. In an autoimmune disease, the ability of the immune system to recognize what is "self," or to turn off attacks that injure it, must be defective.

With most autoimmune diseases, women, particularly premenopausal women, are affected more often than men. In animals with autoimmune diseases, it's usually the females that are most severely affected. As a result, scientists suspect hormones are involved.

Autoimmune diseases run in families, so genes are likely to play a role. Many genetic links to particular diseases have been identified.

Infection with bacteria and viruses may play a role. One reasonable theory is that some autoimmune diseases are not really autoimmune after all. What may happen is that infectious agents we haven't yet discovered infect a particular tissue. The immune system recognizes the invader and attacks it - but injures or kills the tissue in which the invader is living.

Exposure to toxins or certain drugs, long-term stress, aging and pregnancy may make a person more susceptible to autoimmune diseases.

Finally, we're learning that the bacteria that inhabit our bodies right from birth and for the rest of our lives influence the development of our immune systems and some autoimmune diseases.

There are about 80 autoimmune diseases that affect specific tissues or organs of the body. These include multiple sclerosis, Type 1 diabetes and rheumatoid arthritis.

Most autoimmune diseases can't be cured, but they can be controlled to some degree. For example, Type 1 diabetes creates a deficiency of insulin, the hormone required for the proper absorption of sugar. By injecting insulin, the diabetic patient replaces what is lost.

Other treatments aim to prevent or slow organ and tissue damage by reducing inflammation.
