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
Primary Medical Reviewer
Anne C. Poinier, MD - Internal Medicine
Specialist Medical Reviewer
Nancy Ann Shadick, MD, MPH - Internal Medicine, Rheumatology
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