What is the Thymus Gland?

The thymus gland is an organ in the upper chest cavity that processes lymphocytes, a type of white blood cell that fights infections in the body. This organ is part of both the lymphatic system, which makes up a major part of the immune system, and the endocrine system, which includes all glands that produce hormones. The thymus is most important in children and young adults, when it programs lymphocytes to attack antigens, like viruses. People who do not have this gland, or in whom it does not function correctly, usually have compromised immune systems and difficulty fighting disease.

Two irregularly shaped parts make up the thymus, which is located directly just below the throat, behind the sternum. The gland releases a hormone that stimulates the production of a certain type of white blood cell in the bone marrow; these cells, called thymocytes are transported by the bloodstream to the thymus. There, the organ "programs" the cells to attack antigens that invade the body and to not attack normal cells in the body itself. Once matured, these T lymphocytes, or T cells, circulate through the bloodstream and collect in the lymph organs — the spleen and lymph nodes — for future use.

T cells are called into action to fight viruses, tumor cells, and other invaders to help the body fight off disease. They also help in the development of other white blood cells, including B cells, which develop in the bone marrow, and macrophages, which "swallow" foreign cells.

The majority of lymphocyte production happens early in life, so the thymus gland shrinks with age. It's about the size of an apple in children just before puberty, but may become barely discernible from surrounding fatty tissues in the elderly. It is thought that the sex hormones released during adolescence trigger the organ to begin to shut down. Because it's smaller and less active in adults, little was known about the thymus gland until the 1960s, and scientists are still studying exactly how it is related to various diseases and conditions.

If the thymus is removed in infancy or develops improperly, the immune system may be compromised. Much of the body's immune system development happens before birth, so removing the organ even in a young child won't necessarily cause extreme damage to the child's immunity. When the thymus doesn't develop correctly, however, it can cause immune deficiency, making the person much more susceptible to infections.

Cancer is rare in this part of the body, but tumors can develop in the thymus. Called a thymoma, these tumors most often occur in people with other medical conditions including myasthenia gravis and some autoimmune diseases. Symptoms include coughing, wheezing, and chest pain, and treatment can include surgery to remove the tumor along with radiation or hormone therapy. The prognosis for thymoma depends mostly on how far the cancer has progressed.

The thymus is thought to play a role in the development of myasthenia gravis, a condition in which the T cells attack the nerves where they connect to the muscles. The removal of the organ, called a thymectomy, is often performed to relieve the symptoms related to this condition.

Research is being conducted to determine whether or not regenerating the thymus gland or preventing its deterioration could improve immunity in older people. Scientists question whether the organ could play a role in fighting cancer and HIV/AIDS, which directly attacks T cells. Numerous autoimmune diseases such as multiple sclerosis, lupus, and diabetes might also be managed more effectively through better understanding the gland's function.