Thymus is a flat, pinkish-gray organ that plays an important part in the immune system of the body. It is located high in the chest cavity behind the breastbone and extends into the lower neck below the thyroid gland.
The thymus aids in the development of white blood cells called lymphocytes, which help the body fight disease. There are two kinds of lymphocytes, both of which are formed from cells in the bone marrow. Some lymphocytes, called B cells, probably mature in the bone marrow itself (see Immune system (Parts of the immune system) ). The B stands for bone marrow derived. The other lymphocytes travel to the thymus, where they are changed into T cells. The T stands for thymus derived. The thymus produces a substance called thymosin, which scientists believe plays an important part in the change into T cells.
The T cells leave the thymus and inhabit the blood, lymph nodes, and spleen. There, they attack bacteria, cancer cells, fungi, viruses, and other harmful organisms. Killer T cells, also called cytotoxic T cells, find and destroy abnormal or infected cells. T cells also attack organs that have been transplanted from one person to another. This is called transplant rejection. Powerful drugs are needed to prevent T cells from destroying the transplanted organ.
When a person is born, the thymus weighs about 1/2 ounce (15 grams). When a person is 12 years old, the thymus is about twice its original size. At that time, the lymph nodes and the spleen take over the task of producing lymphocytes. The thymus then begins to shrink and produces fewer T cells. By adulthood, the organ has shrunk so much that it may be hard to distinguish from the fatty tissue that surrounds it. Babies born without a thymus may fail to develop a normal immune system.
The thymus may be removed to treat myasthenia gravis, a disease characterized by severe muscle weakness. For reasons not completely understood, thymus removal helps many patients with this disease become stronger. Loss of the thymus has no noticeable effect on the immune system. See Myasthenia gravis.
