thymosin

thymosin [′thī·mə·sən]

(immunology)

Any of a group of hormones secreted by the thymus gland that stimulate lymphocyte production within the thymus and confer on lymphocytes elsewhere in the body the capacity to respond to antigenic stimulation.

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Thymosin

A polypeptide hormone synthesized and secreted by the endodermally derived reticular cells of the thymus gland. Thymosin exerts its actions in several loci: (1) in the thymus gland, either on precursor stem cells derived from fetal liver or from bone marrow, or on immature thymocytes, and (2) in peripheral sites, on either thymic-derived lymphoid cells or on precursor stem cells. The precursor stem cells, which are immunologically incompetent whether in the thymus or in peripheral sites, have been designated as predetermined T cells or T₀ cells, and mature through stages termed T₁ and T₂, each reflecting varying degrees of immunological competence. Thymosin promotes or accelerates the maturation of T₀ cells to T₁ cells as well as to the final stage of a T₂. In addition to this maturation influence, the hormone also increases the number of total lymphoid cells by accelerating the rate of proliferation of both immature and mature lymphocytes. See Immunity