What happens to the joint in rheumatoid arthritis?

Rheumatoid arthritis is an autoimmune disease, meaning that it is caused by the attack of the body's immune system against its own tissues. In rheumatoid arthritis, cells of the immune system are found in large numbers within the inner structures of the joint. When the first immune cells invade the joint, they send out chemical messages through the bloodstream to call in reinforcements. These chemicals induce changes in the tissues around the joint to make it easier for the new immune cells to reach the joint. These chemicals increase blood flow to the region around the joint and make the blood vessels more leaky so that fluid (and immune cells) can leave the blood vessels and travel into the tissues. This response is called an inflammatory response and leaves the joint warm and swollen from the fluid accumulation. It also causes joint pain because of destruction of bone and cartilage tissue in the joint and because pain-causing chemicals are released.

The chemicals that are released by the immune system have additional roles. They encourage the immune cell recruits to divide, further increasing their numbers. These chemicals also induce other cells to divide and enlarge their populations. One of the most striking features of the joint in rheumatoid arthritis is the overgrowth of the cells that line the inside of the capsule surrounding the joint and that secrete the fluid that fills the joint.

Normally, the joint lining (synovium) is a single or few layers thick. In rheumatoid arthritis, the synovium is filled with immune cells, blood vessels, and fibrous cells that form a structure called a pannus. This pannus not only fills the joint space but it also grows so aggressively that when it runs out of space within the joint it erodes through the cartilage and surrounding bone as it expands. The pannus is also responsible for the joint warmth, swelling, and pain associated with rheumatoid arthritis.

Reacting against the tissues of the joint as if they were foreign substances meant to be neutralized, the cells of the immune system attack and destroy both cartilage and bone. In an attempt to heal the wounds caused by the attack, the cartilage and bone cells initially try to correct the damage by dividing and growing, creating new tissue. This new joint tissue does not form correctly, however, and often is the wrong shape or is less functional than the original joint.