Rheumatoid arthritis occurs when the immune system, the body's defense against invading viruses and bacteria, starts to attack the membranes that surround the joints called the synovium. Immune system cells attack these tissues causing inflammation, swelling, pain, and stiffness.
While it is clear that an autoimmune reaction causes rheumatoid arthritis, it is not clear what causes that reaction to occur in certain people and not others. It is likely that several factors interact to cause the disease including genetics, lifestyle, and environmental factors such as exposure to certain viruses.
Continued on page 4: Who is at risk for rheumatoid arthritis?