Rheumatoid arthritis. Illustration of a coronal (frontal) section through a knee joint, showing the effects of rheumatoid arthritis. This is a condition in which the body's immune system attacks the tissues of the joints. It is characterized by swelling and inflammation (red dots) of the synovial membrane (pink). The joint cartilage (blue) is extremely eroded here, with vascular infiltration and proliferation into the bone tissue (red areas in both the femur and tibia), destruction of the bony matrix, and pannus formation (fibrovascular or granulation tissue). The joint capsule (orange) is also shown. For the same joint with osteoarthritis, see image C029/0669.