Table of Contents
Definition of Thymus
A structure of lymphatic tissue located in the upper central chest, behind the sternum (breastbone) midway between top of the heart and the sternal notch at the base of the throat. The thymus is fairly large at birth, spreading in a loosely shaped “H” across the great vessels that emerge from the heart.
The tissue of the thymus sometimes extends upward to make contact with the thyroid glad and downward to drape over the heart’s upper chambers, the atria. Occasionally fragments of thymic tissue exist as unattached, isolated lobules typically remaining in the upper chest and lower neck.
Function of Thymus
The thymus is most active in youth, serving as the incubator in which T-cells, the lymphocytes crucial for the body’s defense against infection, mature and differentiate (acquire their functional characteristics). Typically enlarges as a child approaches puberty, its peak time of activity, then begins to recede. By midlife little more than strands of thymic tissue remain.
The thymus also produces hormones-key among them being thymosin, thymulin, thymopoietin, and thymocyte humoral factor-that regulate T-cell maturation.
Status Lymphaticus: Enlarged Thymus
In the 1940s and 1950s, conventional medical wisdom blamed the large thymus of childhood for unexplained sudden death in children, conveying upon the condition the diagnostic label status lymphaticus. Radiation therapy to shrink the thymus became the prevailing treatment. By the 1960s doctors recognized the thymus was normally large in children and abandoned the diagnosis and its treatment.
Researchers believe the thymus has other functions related to immune response, though remain unable to determine their precise mechanisms. An infant born without a thymus has no ability to develop an immune system; this congenital anomaly is nearly always fatal in infancy.
There also appears to be a correlation between the thymus and myasthenia gravis, an autoimmune disorder in which the immune system produces antibodies that target acetylcholine, the neurotransmitter that facilitates nerve signals traveling between nerve cells and muscle cells. An enlarged thymus is common in people who have myasthenia gravis and removing it (thymectomy) often results in dramatic improvement in the condition.
The thymus may have a role in other autoimmune disorders such as systemic lupus erythematosus (sle) and Graves’s disease. Thymoma and thymic carcinoma are two forms of cancer that may develop in the thymus. Cancer of the thymus is uncommon.
For further discussion of blood and lymph structure and function please see the overview section “The Blood and Lymph.”
See also HIV/AIDS.
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