The cause of rheumatoid arthritis (RA) is not fully understood. Genes play a role in rheumatoid arthritis, but experts do not know exactly what that role is. For most people with RA, the disease does not run in their families and they do not pass it along to their children. One or more genes may make it more likely that the body's immune system will attack the tissues of the joints. This immune response may also be triggered by bacteria, a virus, or some other foreign substance.

The abnormal immune response causes ongoing inflammation of the tissues lining the joint, a breakdown of cartilage, and loosening of the ligaments and tendons supporting the joint. Ongoing inflammation also causes the membrane that lines the joint (synovium) to grow into a thick, abnormal tissue called pannus. These processes result in destruction of the cartilage, the underlying bone surrounding the joint, ligaments, and tendons and can eventually lead to deformed joints.