In rheumatoid arthritis, the body attacks its own joints. White blood cells, agents of the immune system, travel to the synovium and cause inflammation (synovitis). The inflamed synovium causes warmth, redness, swelling, and pain.
During the inflammatory process, the synovium thickens and causes the joint to swell. As rheumatoid arthritis progresses, abnormal synovial cells invade and erode cartilage and bone within the joint. Surrounding muscles, ligaments, and tendons weaken.