Antigens: Foreign invaders in the body, such as bacteria, fought by one’s antibodies when the body’s immune system is working effectively.

Arthralgia: Pain in a joint in the absence of signs or symptoms of inflammation.

Arthritis: A disease characterized by the inflammation of one or more joints; there are more than 100 different types. The joints may become warmer than usual and show redness, swelling, pain and tenderness.

Arthrodesis: A process through which portions of an otherwise very unstable and deformed joint are surgically removed and the joint is fused and permanently locked into its most useful position.

Arthrogram: An X-ray technique for examining joints. A contrast medium (either air or a liquid, opaque to X-ray) is injected into the joint space, allowing its outline and contents to be traced accurately. The machine for taking out the arthrogram is called arthroscope.

Arthroplasty: A process of joint reconstruction that involves surgically rebuilding or modifying a joint or replacing it with an artificial joint.

Articular Cartilage: See Cartilage.

Autoimmune Disease: A disease in which a person’s antibodies, which normally attack only “nonself” or foreign antigens, attack his or her own body. Some researchers say that systemic lupus erythematosus (SLE) may be one such disease.

Azathioprine: The generic name of an immunosuppressant drug that slows the progression of rheumatoid arthritis in some patients. Short-term side effects seem relatively uncommon, though it has not yet been determined what effects the drug may have in the long term.

Bacteria: Bacteria are tiny single-celled or noncellular organisms. They exist in great variety. Some need air to exist, while, others do not; they may subsist through a great range of temperatures and require a variety of nutrients. While some are utterly harmless or even beneficial, others are extremely deadly. Bacteria may invade the joints and produce infectious arthritis.