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Development and functional properties of thymic and extrathymic T lymphocytes.

Heinonen KM, Perreault C

Institute for Research in Immunology and Cancer; Department of Medicine, Universite de Montreal, Montreal, QC, Canada H3C 3J7.

Although vertebrate hematopoiesis is known to occur in a variety of locations, the differentiation of T lymphocytes takes place only in the thymus. Conversely, the only known function of the thymus is to produce T lymphocytes. As thymic function progressively diminishes with age in all individuals, and decreased thymic capacity has been linked to an impaired resistance to infections and cancer, increasing thymic function and/or extrathymic T-cell development could thus play an important role in health management in elderly subjects. However, it is currently unclear why the thymus is essential for T-cell development and precisely what is necessary to make a functional T lymphopoietic organ. In this review, we shall discuss the current literature on the similarities and differences between thymic and extrathymic T cells. A major conclusion is that innate T cells can be generated extrathymically, but only the thymus can support the development of classic adaptive T cells. Functional differences between thymic and extrathymic T cells provide a rationale for conservation of the thymus as the sole primary T lymphoid organ. These differences also have implications for the management of thymus involution.

Crit. Rev. Immunol. 2008;28(5):441-66.

Pubmed ID: 19166388

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