AskDefine | Define thymus

Dictionary Definition

Thymus

Noun

1 large genus of Old World mints: thyme [syn: genus Thymus]
2 a ductless glandular organ at the base of the neck that produces lymphocytes and aids in producing immunity; atrophies with age [syn: thymus gland]

User Contributed Dictionary

see Thymus

English

Pronunciation

Noun

  1. In the context of "anatomy|immunology": A ductless gland, consisting mainly of lymphatic tissue, located behind the top of the breastbone. It is most active during puberty, after which it shrinks in size. It plays an important role in the development of the immune system and produces lymphocytes.

Translations

ductless gland located behind the breastbone
  • Czech: brzlík
  • Finnish: kateenkorva
  • Italian: timo
  • Polish: grasica
  • Swedish: bräss, thymus

Derived terms

See also

Swedish

Noun

  1. In the context of "anatomy|immunology|lang=sv": thymus

Synonyms

Extensive Definition

In human anatomy, the thymus is an organ located in the upper anterior portion of the chest cavity just behind the sternum. Hormones produced by this organ stimulate the production of certain infection-fighting cells. It is of central importance in the maturation of T cells.

History

The thymus was known to the Ancient Greeks. Galen was the first to note that the size of the organ changed over the duration of a person's life.
Due to the large numbers of apoptotic lymphocytes, the thymus was originally dismissed as a "lymphocyte graveyard", without functional importance. The importance of the thymus in the immune system was discovered in 1961 by Jacques Miller, by surgically removing the thymus from three day old mice, and observing the subsequent deficiency in a lymphocyte population, subsequently named T cells after the organ of their origin. Recently advances in immunology have allowed the fine dissection of the function of the thymus in T cell maturation.

Function

In the two thymic lobes, lymphocyte precursors from the bone-marrow become thymocytes, and subsequently mature into T cells. Once mature, T cells emigrate from the thymus and constitute the peripheral T cell repertoire responsible for directing many facets of the adaptive immune system. Loss of the thymus at an early age through genetic mutation or surgical removal results in severe immunodeficiency and a high susceptibility to infection.
The stock of T-lymphocytes is built up in early life, so the function of the thymus is diminished in adults. It is, therefore, largely degenerated in elderly adults and is barely identifiable, consisting mostly of fatty tissue.
The ability of T cells to recognize foreign antigens is mediated by the T cell receptor. The T cell receptor undergoes genetic rearrangement during thymocyte maturation, resulting in each T cell bearing a unique T cell receptor, specific to a limited set of peptide:MHC combinations. The random nature of the genetic rearrangement results in a requirement of central tolerance mechanisms to remove or inactivate those T cells which bear a T cell receptor with the ability to recognise self-peptides.

Phases of thymocyte maturation

The generation of T cells expressing distinct T cell receptors occurs within the thymus, and can be conceptually divided into three phases:
  1. A rare population of hematopoietic progenitors enters the thymus from the blood, and expands by cell division to generate a large population of immature thymocytes.
  2. Immature thymocytes each make distinct T cell receptors by a process of gene rearrangement. This process is error-prone, and some thymocytes fail to make functional T cell receptors, whereas other thymocytes make T cell receptors that are autoreactive. . Growth factors include thymopoietin and thymosin.
  3. Immature thymocytes undergo a process of selection, based on the specificity of their T cell receptors. This involves selection of T cells that are functional (positive selection), and elimination of T cells that are autoreactive (negative selection).
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