The Thymus is a gland-like organ of the immune system formed by T lymphocytes, which are the cells in charge of cellular immunity, responding with activation of some cells to fight infections. The response of the immune system is very important in hematology in the rejection of hematopoietic progenitor transplants.
The thymus: what is it and what is it used for?
The thymus is an organ of the immune system, in the form of a gland. It was known in Ancient Greece by the origin of the word Thymos, meaning “heart or soul”, possibly because of its location in the chest, “near where desires and emotions are subjectively felt”.
Galen was the first to realize that the size of this organ changed in size throughout life. Initially it was considered a “lymphocyte graveyard”, with no functional value. However, its importance in the immune system was discovered in 1961 by Jacques Millar, when he surgically removed the thymus from a 3-day-old mouse and observed the subsequent deficiency in the number of lymphocytes, which were later called T cells, a name synonymous with “derived thymus”.
What the thymus gland looks like
The thymus is a two-lobed organ located in the upper chest cavity, partially within the neck region. It is located above the cardiac pericardium, in front of the aorta, between the lungs, below the thyroid gland and behind the sternum. The thymus has a thin covering called the capsule. It is made up of three types of cells:
- Epithelial cells. These are compact cells that give the thymus its shape and structure.
- Lymphocyte. These are immune cells that protect against infection and stimulate the immune response.
- Kulchitsky cells. These are cells that release hormones.
Lobules are made up of many smaller divisions called lobules. A lobule consists of an inner area called the medulla, and a peripheral region called the cortex:
- Cortex region: contains immature T lymphocytes, so these cells do not yet have the immunological ability to distinguish the body’s own cells from foreign cells.
- Medulla region: contains mature T lymphocytes. These cells have the ability to identify self cells from foreign cells by differentiating into specialized T lymphocytes. T lymphocytes originate from hematopoietic stem cells in the bone marrow and migrate in an immature state to the thymus via the bloodstream.
Function of the thymus
The main function of the thymus is to mature T lymphocytes. Once mature, they leave the thymus and are transported through the blood to the lymph nodes and spleen.
It should be noted that T lymphocytes are the cells of the immune system responsible for cell-mediated immunity, which is an immune response that triggers the activation of specific immune cells to fight infection. Their cell membrane contains a series of proteins capable of recognizing different types of allergens (substances that elicit an immune response). T lymphocytes are differentiated into 3 types in the thymus:
- Cytotoxic T cells. They bind directly to the antigens of the pathogen and eliminate it.
- Helper T cells. They precipitate the production of antibodies by B lymphocytes and also produce substances that activate other T cells.
- Regulatory T cells. They are also called suppressor T cells because they suppress the response of B cells and other T cells to antigens.
The thymus produces a number of hormone-like proteins that help mature and differentiate T cells. Some of these hormones, such as Tymopoetin and Thymulin, induce the differentiation of T lymphocytes, enhancing their immune function. In fact, Thymopoetin enhances the immune response.
On the other hand, the thymus as a hormonal gland also stimulates other glands. An example is the hypothalamic-pituitary axis, which stimulates the release of growth hormones, luteinizing hormones, prolactin and gonadotropin, and the adrenal gland, stimulating the production of ACTH.
Immunological importance in hematology
The response of the immune system is of vital importance in hematology for the rejection of hematopoietic progenitor transplants, as in graft engraftment, acute or hyperacute rejection, and in graft-versus-host disease.
Hematopoietic transplants consist of the infusion of hematopoietic progenitors from peripheral blood after their mobilization and conditioning to obtain an adequate amount to ensure the success of the therapy. This type of transplants are used to replenish the bone marrow (factory where all blood components originate) when it is depleted by aggressive radiotherapy or chemotherapy treatments, as the only curative treatment option in malignant tumor hematopathies such as Leukemia, Lymphoma, Myeloma or congenital bone marrow aplasia.