The thymus gland is a small organ, part of the endocrine system, at the back of the sternum, between the lungs. It gets smaller with age, and is gradually replaces with adipose tissue. When it is working properly, it creates and trains immune cells, though fewer and more slowly as more and more of the original tissue gets replaced.
It was named for the herb thyme, which ancient doctors thought the organ resembled. however, it was believed to be only a repository of dead immune cells until 1961, when the gland’s function was discovered. Because of the role of the thymus in the immune response, these antibodies are called T-lymphocytes.
The T-lymphocytes are the part of the immune system that directly attack intruders. These cells identify their targets by their proteins. The thymus is responsible for eliminating the T-lymphocytes that are programmed to go after the body’s own cells. It produces the same protein patterns as the body’s organs and eliminates antibodies that respond.
Dysfunction in the thymus can result in one of a small number of autoimmune diseases, of which myasthenia gravis is by far the most common. It is believed that myasthenia results from abnormal or excessive growth of the thymus, which leads to the production of too many T-lymphocytes. A small number of myasthenia cases are caused by a thymoma, a usually benign tumor on the thymus. In these cases, removal of the tumor, or even the entire gland, may be necessary.
Although the exact mechanism by which thymus activity leads to myasthenia is unknown, scientists have some clues. One common hypothesis is that the thymus gives erroneous instructions to the T-lymphocytes. The myoid cells in the thymus that are supposed to catch T-lymphocytes that target muscles are misaligned, causing the antibodies to think the acetylcholine receptor sites they attack are invading foreign cells. As a result, the T-lymphocytes attack the receptors, destroying as many as 80 percent of them throughout the body and rendering the muscles unable to respond to neural signals.
Research is ongoing into the role of the thymus in myasthenia gravis and what this might mean for treatment. If the exact link can be determined, doctors may be able to treat the condition with minimal side effects.