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thymus

thymus is a topic covered in the Taber's Medical Dictionary.

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(thī′mŭs )

[Gr. thymos]
A primary lymphoid organ located in the anterior mediastinum. The thymus comprises two fused lobes, the right larger than the left. The lobes are partially divided into lobules, each of which has an outer cortex packed with immature and developing T lymphocytes (thymocytes) and an inner medulla containing a looser arrangement of mature T lymphocytes.


THYMUS
The thymus is the primary site for T-lymphocyte differentiation; here, T lymphocytes acquire their range of antigen receptors. During the prenatal period, lymphoid stem cells migrate from the bone marrow to the thymus, filling the cortex of the lobules. Developing thymocytes acquire their characteristic CD surface antigens and their binding receptors. As the thymocytes then move from the cortex into the medulla of the lobules, some are protected but many undergo cell death in a process that culls out those reactive to autoantigens. Less than 5% of the thymocytes mature into T cells that pass out of the lobules and migrate to the spleen, lymph nodes, and other lymphoid tissue, where they control cell-mediated immune responses. The thymus produces at least four hormones: thymopoietin, thymulin, thymus humoral factor, and the thymosins.

At birth, the thymus weighs 10 to 15 g; by puberty, it weighs about 20 g. After this, the cortical regions of the thymus shrink and become replaced by adipose tissue, although the thymus continues to produce hormones and some thymocytes into old age.

PATHOLOGY
Lack of a thymus or thymus hypoplasia is one component of DiGeorge syndrome, which is marked by severe lack of cell-mediated immunity; removal of the thymus of an adult is less catastrophic but leads to a less effective response to new antegens. Thymic hyperplasia results from the growth of lymph follicles containing both B lymphocytes and dendritic cells. It is found in myasthenia gravis and, occasionally, in other autoimmune diseases, e.g., Graves disease, rheumatoid arthritis, and systemic lupus erythematosus. Thymomas involve only the thymic epithelial cells. Other tumors, including those associated with Hodgkin's disease and lymphomas, involve thymocytes.


THYMUS IN A YOUNG CHILD

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Citation

Venes, Donald, editor. "Thymus." Taber's Medical Dictionary, 23rd ed., F.A. Davis Company, 2017. Nursing Central, nursing.unboundmedicine.com/nursingcentral/view/Tabers-Dictionary/767874/all/thymus.
Thymus. In: Venes D, ed. Taber's Medical Dictionary. 23rd ed. F.A. Davis Company; 2017. https://nursing.unboundmedicine.com/nursingcentral/view/Tabers-Dictionary/767874/all/thymus. Accessed June 24, 2019.
Thymus. (2017). In Venes, D. (Ed.), Taber's Medical Dictionary. Available from https://nursing.unboundmedicine.com/nursingcentral/view/Tabers-Dictionary/767874/all/thymus
Thymus [Internet]. In: Venes D, editors. Taber's Medical Dictionary. F.A. Davis Company; 2017. [cited 2019 June 24]. Available from: https://nursing.unboundmedicine.com/nursingcentral/view/Tabers-Dictionary/767874/all/thymus.
* Article titles in AMA citation format should be in sentence-case
TY - ELEC T1 - thymus ID - 767874 ED - Venes,Donald, BT - Taber's Medical Dictionary UR - https://nursing.unboundmedicine.com/nursingcentral/view/Tabers-Dictionary/767874/all/thymus PB - F.A. Davis Company ET - 23 DB - Nursing Central DP - Unbound Medicine ER -