A joint is where two bones meet. Joints allow movement and flexibility of various parts of the body. The movement of the bones is cased by muscles which pull on tendons that are attached to bone.
Cartilage covers the end of bones. Between the cartilage of two bones which form a joint there is a small amount of thick fluid called synovial fluid. This fluid 'lubricates' the joint which allows smooth movement between the bones.
The synovial fluid is made by the synovium. This is the tissue that surrounds the joint. The outer part of the synovium is called the capsule. This is tough, gives the joint stability, and stops the bones from moving 'out of joint'. Surrounding ligaments and muscles also help to give support and stability to joints.
RA is thought to be an 'autoimmune disease'. The immune system normally makes antibodies to attack bacteria, viruses, and other 'germs'. In people with autoimmune diseases, the immune system makes antibodies against tissues of the body. It is not clear why this happens. Some people seem to have a tendency to develop autoimmune diseases. In such people, something might trigger the immune system to attack the body's own tissues. The 'trigger' is not known.
In people with RA, antibodies are formed against the synovium (the tissue that surrounds each joint). This causes inflammation in and around affected joints. Over time, the inflammation can cause damage to the joint, the cartilage, and parts of the bone near to the joint.
The most commonly affected joints are the small joints of the fingers, thumbs, wrists, feet, and ankles. However, any joint may be affected. The knees are quite commonly affected. Less commonly the hips, shoulders, elbows, and neck are involved. It is often symmetrical. So, for example, if a joint is affected in a right arm, the same joint in the left arm is also often affected.
In some people, just a few joints are affected. In others, many joints are involved.