The thymus gland is located in the upper thorax behind the sternum, but below the thyroid gland.
It has two lobes, each divided into lobules by a partition called a septum or wall. Each lobule consists of a densely packed outer cortex and a paler center, called the medulla.
Lymphocytes divide in the cortex, and, as the T-cells mature, they migrate into the medula and eventually enter one of the blood vessels in that region.
Other cells within the lobules produce hormones, collectively known as thymosins. Thymosins promote the maturation of lymphocytes within the gland, as well as the growth and activity of lymphocytes throughout the body. When the gland begins to shrink, the amount of thymosins and T-cells that are produced also is reduced. Because of these changes, the defensive mechanisms of the body diminish with age.
The thymus gland is most active during early life, playing a critical role in the development of a child’s immune system before birth and for a time thereafter. Usually by the age of two, the thymus gland has reached its maximum size (weighing about 30 to 40 grams or 1.06 to 1.41 ounces) with the immune system becoming fully functional.
Because of this, vaccinations before the age of two are not really necessary since these young immune systems are not mature enough to handle the strength of a vaccine. (see the Vaccines section in Nature’s Pharmacy: Evidence-based Alternatives to Drugs)
After puberty, the gland begins to shrink and is replaced by connective tissue and fat. The main function of the thymus gland is in the processing and maturation of special lymphocytes called T-cells.