- Related Topics:
- human endocrine system
- Endocrine system
- lymphoid tissue
- On the Web:
- National Center for Biotechnology Information - Anatomy, Head and Neck, Thymus (Nov. 01, 2026)
thymus
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What is the thymus gland?
The thymus is a pyramid-shaped lymphoid organ located beneath the breastbone, near the heart. Its primary role is to facilitate the maturation of T cells, which are lymphocytes that determine the specificity of immune responses to foreign substances in the body.
The thymus grows rapidly, attaining its greatest size during fetal life and the first years after birth, after which it begins to shrink slowly at puberty. This shrinking continues throughout life. The thymus is divided into two lobes and smaller lobules and is composed of lymphocytes and reticular cells. Unlike other lymphoid organs, the thymus doesn't have lymphatic vessels draining into it. The organ's removal in newborns can cause depletion of T cells and failure of the immune system.
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Where is the thymus gland located in the body?
The thymus, a pyramid-shaped lymphoid organ, is located immediately behind the sternum (breastbone) at the level of the heart. It consists of two lobes divided into smaller lobules and is covered by a connective-tissue capsule.
The thymus facilitates the maturation of T cells, which determine the specificity of immune responses. T cell differentiation occurs in the outer cortex of the thymus. As T cells develop, they learn to distinguish between the body’s own components and foreign substances. The thymus grows rapidly, reaching its greatest size during fetal life and early childhood, then shrinks slowly after puberty.
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What is the main function of the thymus gland?
The thymus is a lymphoid organ situated beneath the breastbone that facilitates the maturation of T cells, which determine the specificity of immune response to antigens in the body. T cells, natural killer cells and B cells are types of lymphocytes in the thymus.
The thymus distinguishes between the body’s own components and foreign substances. T cells produced in the thymus are transported to the lymphoid tissues and are crucial elements in the development of immunity. The thymus shrinks with age, which is thought to be the reason T-cell production decreases.
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Why is the thymus important for the immune system?
The thymus is a lymphoid organ that produces T cells, which are lymphocytes that determine the specificity of immune responses to foreign substances in the body. Located behind the sternum, the thymus facilitates the maturation of T cells, enabling them to distinguish between the body's own components and foreign substances.
The thymus is essential for immunological defense. T cells produced in the thymus migrate to the lymphoid tissues and are crucial elements in the development of immunity. The organ destroys lymphocytes that would react against the individual’s own tissues. Autoimmune diseases, like DiGeorge syndrome and Nezelof disease, can result from the failure of the thymus to develop, which leads to a reduction in T-cell numbers.
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How does the thymus change as a person grows older?
The thymus, a lymphoid organ located behind the sternum, facilitates the maturation of T cells, which are essential for immune response. Unlike other lymphatic structures, the thymus grows rapidly, reaching its greatest size during fetal life and early childhood.
However, at puberty, the thymus begins to shrink gradually throughout the rest of a person's life, a process called involution. During this process, the cortex becomes thin, lymphocytes disappear, and fat tissue replaces them. The decline of the thymus is considered the reason T-cell production decreases with age. Despite the shrinkage, bits of thymus tissue remain to maintain its function.
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What happens if the thymus does not work properly?
The thymus is a lymphoid organ that facilitates the maturation of T cells, which determine the specificity of immune response to foreign substances in the body. Removal of the thymus in newborns leads to a depletion of T cells in the blood and lymphoid tissue. This failure of the immune system causes a gradually fatal wasting disease. The individual becomes less able to reject foreign-tissue grafts or to make antibodies to certain antigens. Moreover, certain parts of the white pulp of the spleen and lymph nodes are much reduced in size.
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thymus, pyramid-shaped lymphoid organ that, in humans, is immediately beneath the sternum (breastbone) at the level of the heart. The organ is called thymus because its shape resembles that of a thyme leaf. The primary function of the thymus is to facilitate the maturation of lymphocytes known as T cells, or thymus-derived cells, which determine the specificity of immune response to antigens (foreign substances) in the body.
Unlike most other structures in the lymphatic system, the thymus grows rapidly and attains its greatest size relative to the rest of the body during fetal life and the first years after birth. Thereafter, it continues to grow, but more slowly than the other organs. At the onset of puberty, the thymus begins a slow process of shrinking. This gradual diminution in size continues for the rest of the individual’s life.
The thymus is divided into two lobes, lying on either side of the midline of the body, and into smaller subdivisions called lobules. It is covered by a dense connective-tissue capsule, which sends fibers into the body of the thymus for support. The thymus tissue is distinguishable into an outer zone, the cortex, and an inner zone, the medulla.
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The organ is composed principally of two types of cells, called, respectively, lymphocytes (specifically, T cells, B cells, and natural killer cells) and reticular cells. The reticular cells form a loose meshwork, as in a lymph node, while the spaces between them are packed with lymphocytes. The cortex, characterized by its heavy lymphocyte concentration, is the site of much lymphocyte proliferation, especially of T cells, and the site of T cell differentiation. Proliferation of lymphocytes in the thymus is distributed evenly throughout the cortex, instead of in germinal centers, as occurs in other lymphoid tissue. Some of the T cells that are produced in the cortex migrate to the medulla, where they enter the bloodstream through the medullary veins, adding to the lymphocytes seen in the peripheral blood and the lymphoid organs.
The thymus differs structurally from other lymphoid organs in that it does not have lymphatic vessels draining into it. It is not a filter like the lymph nodes, which are situated so that microorganisms and other antigens are exposed to their cells. The thymic lymphocytes are sealed off from the rest of the body by a continuous layer of epithelial (covering) cells that entirely surround the organ. While thus sequestered, the lymphocytes differentiate, or acquire the capabilities to perform specialized tasks. (It has been suggested that hormonal functions of the thymus aid in this differentiation.) Of these specialized lymphocytes, helper T cells work synergistically with the thymus-independent lymphocytes (B cells) to produce antibodies. Cytotoxic T cells directly attack invading microorganisms and foreign tissue, including organ transplants.
The functions of the thymus that have so far been observed relate chiefly to the newborn. Removal of the organ in the adult has little effect, but, when the thymus is removed in the newborn, T cells in the blood and lymphoid tissue are depleted, and failure of the immune system causes a gradually fatal wasting disease. The individual whose thymus has been removed at birth is less able to reject foreign-tissue grafts or to make antibodies to certain antigens. Moreover, certain parts of the white pulp of the spleen and lymph nodes are much reduced in size. These results demonstrate that the T cells produced in the thymus and transported to the lymphoid tissues are crucial elements in the development of immunity.
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It is known that most of the lymphocytes that are produced in the thymic cortex die without leaving the organ. Since those T cells that do leave the thymus are equipped to react against foreign antigens, it is assumed that the thymus destroys lymphocytes that would engage in an autoimmune reaction—that is, would react against the individual’s own tissues.
During the involution, or shrinking, of the thymus, the cortex becomes thin. Lymphocytes disappear and are replaced by fat tissue from the partitions between the lobules. The process of involution is never complete, and the bits of thymus tissue that remain are probably sufficient to maintain its function.