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