Immune System

A system of cells, tissues, and organs that work together to protect the body against “foreign” invaders such as bacteria, viruses, fungi, parasites, and cancerous cells.

The immune system contains a number of components that work in concert to ward off invaders and keep us healthy. Perhaps the most important component is a type of white blood cell called lymphocytes. Lymphocytes are produced in the bone marrow, the spongy tissue found inside of the bones. The thymus, a tiny organ located near the heart, is responsible for teaching specific lympocytes, called T lymphocytes, how to differentiate “self” (healthy cells in the body) from “non-self” (invaders or cancerous cells). T cells regulate the immune response and can destroy “non-self” cells directly. Other components of the immune system include the following:

• Antibodies, Y-shaped proteins made by the white blood cells, which respond specifically to different proteins (antigens) in bacteria, viruses, or toxins.
• The lymphatic system, comprised of lymph nodes and their blood vessels, which filter bacteria and viruses from lymphatic fluid. (Swollen lymph nodes can be a sign of infection.)
• Tonsils, located in the throat, which are full of lymphocytes and can activate the immune system quickly if a pathogen enters through the mouth.
• The spleen, which stores various immune cells and blood platelets and removes red blood cells from the bloodstream.

Disorders of the immune system can cause a number of medical conditions. A deficiency in the immune response due to a genetic disorder, an acquired conditions such as AIDS, or the use of immunosuppressive medicines may leave the body susceptible to potentially life-threatening infections.

In autoimmune diseases, the immune system mistakenly attacks the body’s own tissues as if they were foreign. There are more than 80 autoimmune diseases, including the following:

• Hashimoto thyroiditis, in which the immune system attacks the thyroid gland.
• Type 1 diabetes, in which T cells attack the insulin-producing beta cells of the pancreas.
• Rheumatoid arthritis, in which T cells launch an attack on the synovium, the lining of the joints.
• Systemic lupus erythematosus, in which the immune system attacks multiple tissues throughout the body.

Understanding the autoimmune nature of these conditions has allowed scientists to develop drugs that specifically block these processes.

Robert S. Dinsmoor is a medical writer and editor based in Massachusetts.

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