The thymus is a gland located in the anterior portion of the chest cavity, in front of the trachea and its main function is to ensure the maturation of T cells. T cells are a type of white blood cells that play a key role in the immune system. This gland develops until it reaches the thyroid gland with its upper end and the fourth rib cartilage with its lower end.
What is the thymus gland?
The thymus is the gland in which the maturation of T lymphocytes occurs. It is composed of two lobes of different sizes connected together by connective tissue and covered by a thin fibrous capsule. Within each of the lobes there is a superficial region of tissue referred to as the cortex as well as a deep region known as the medulla. Some of the T cells produced in the cortex travel to the medulla from where they enter the bloodstream through the medullary veins and become an addition to the lymphocytes that are present in the peripheral blood and the lymphoid organs.
The outer layer of the gland is full of thymocytes or cells that represent different stages of T cells maturation between lymphoid "newborn" cells from the bone marrow and immature T cells. The inner part of this gland consists of two medullary regions, which continue into one another, and it also contains mature lymphocytes and epithelial cells that secrete hormones (timostimolina, thymopoietin, thymopentin, thymosin, etc.).
When T cells fully mature they flow into the bloodstream and migrate to the lymph glands and the spleen.
The production of T cells from the thymus gland begins in embryonic life, around the eighth week of gestation. The activity of this gland, which is essential for the immune system, decreases with time: it begins to decrease during puberty and shrinks towards adulthood causing a small amount of T cells to be produced. The renewal of these cells is continuously performed in the peripheral lymphoid organs.
The main arterial supply of the thymus gland is provided through the anterior intercostal arteries and small branches from the internal thoracic arteries. The venous blood drains into the left branchiocephalic and internal thoracic veins.
What function does the thymus gland serve?
The main function of the thymus gland is to receive immature T cells from the bone marrow and to ensure the maturation of those cells. Once they acquire immune competence through complete maturation, these cells migrate through the bloodstream in the peripheral lymphoid centres (spleen and lymph nodes). From the peripheral centres they can then move to the sites where there is a presence of antigens (i.e. those substances that can induce reactions of the immune system) and release cytotoxic factors (that is toxic to cells) and activators of macrophages (phagocytic cells).
Moreover, the thymus gland is responsible to train or program the T cells not to attack the body’s own tissues. This is referred to as autoimmunity, meaning that the T cells may act as self antigens and react against the body’s tissue.
