Rheumatoid arthritis is joint pain and stiffness caused by an immune reaction to joint tissue linings. It damages the joints and can even destroy them. Rheumatoid arthritis usually affects the hands, feet, knees, shoulders, and wrists, but it can develop in any joint. There is no cure, but treatment improves signs and symptoms and slows joint damage. Treatment is with drugs that alter the body's immune reactions. Abatacept is a new drug that targets specific components of the immune system. Abatacept inhibits the specific immune cells that cause rheumatoid arthritis and has been shown to improve symptoms in people with rheumatoid arthritis. It has not been directly compared with drugs that are commonly used to treat rheumatoid arthritis, such as methotrexate.