Rheumatoid arthritis is thought to be an autoimmune disease. That is, the body's immune defenses — the defenses in the body that work to fight off invaders (such as bacteria and viruses) — turn against healthy tissue. More specifically, the white blood cells that normally seek out and attack foreign invaders move into the joint and attack healthy connective tissue. Why the immune system behaves this way is unknown. Some research suggests that a virus or other infection triggers the faulty immune response, but no single infectious agent has been identified as the cause in all patients.

Several research studies suggest a genetic aspect to rheumatoid arthritis. For example, rheumatoid arthritis occasionally runs in families. If one identical twin has rheumatoid arthritis, the other is much more likely to develop the disease when compared with a nonidentical twin or a non-twin sibling.

Researchers also have shown that certain genetic markers, including one called human lymphocyte antigen (HLA)-DR4, are more common among people with rheumatoid arthritis than among those without the disease. Other genes that appear to influence the development and severity of rheumatoid arthritis also have been identified. Research findings suggest that the presence of an abnormal gene may increase the risk of an autoimmune process in rheumatoid arthritis, perhaps in conjunction with an environmental trigger, such as a virus or smoking.

Although rheumatoid arthritis most often occurs between the ages of 20 and 50, it also can affect children and the elderly. The disease is found in all ethnic groups worldwide. In the United States, approximately 1 percent of the population (more than 2 million people) have rheumatoid arthritis. Of those affected, 75 percent are women.

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