Rheumatoid arthritis is a chronic inflammatory disease that causes the body's immune system to attack the joints. In some respects, it behaves similarly to an allergy, in which the body attacks a substance it perceives as dangerous. The joints of the body are the intersections where two bones interact with one another. The ends of these bones are covered with cartilage, a smooth, durable substance that allows bones to glide over each other with minimal friction.
Cartilage also acts as a cushioning device to absorb forces applied to the joints.
A structure called the joint capsule surrounds, stabilizes and protects the joint. Within the capsule is a synovial membrane that contains synovial fluid, a viscous substance similar to oil that transports nutrients to the cartilage and keeps its surface well lubricated so the joints can work smoothly. In rheumatoid arthritis, an autoimmune defect causes healthy cells to be attacked. This damage then causes the release of enymes that attack the surrounding tissues. One of these structures is the synovium in the joint.