The smooth lining of the membranes (thin layer of cells) that surround our joints is called the synovium. A flexible joint is lined by a synovial membrane. The synovium produces a clear substance - synovial fluid - which lubricates and nourishes the cartilage and bones inside the joint capsule.
When the immune system attacks the synovium, rheumatoid arthritis may occur. Antibodies attack the synovium, leaving it sore and inflamed - the synovium becomes thicker and may eventually invade and destroy cartilage (the stretchy connective tissue between bones) and bone inside the joint. The joint is held together by tendons (tissue that connects bone to muscle) and ligaments (tissue that connects bone and cartilage). These tendons and ligaments weaken and stretch, and the joint eventually loses its shape and configuration. The joint may eventually be completely destroyed.
Nobody really knows what starts off this process. Experts say some people are genetically predisposed to environmental factors that may trigger rheumatoid arthritis, such as some bacteria or viruses. However, this is a theory which has not been proven.
A knee joint with osteoarthritis (left) and rheumatoid arthritis (right)
Rheumatoid arthritis is an autoimmune condition
An autoimmune condition is an illness that develops when the body tissues are attacked by the immune system. The immune system is a sophisticated system within our bodies, designed to seek out and destroy undesirable invaders, such as infectious agents. A person with an autoimmune disease has unusual antibodies in their blood that attack their own (good) body tissues.
In the case of rheumatoid arthritis, the immune system sends antibodies to the lining of the joints (the synovium) - they attack the tissue surrounding the joint, instead of harmful bacteria or viruses.