Hematopoietic cell growth, migration

Immature T cells produced in the bone marrow ultimately migrate to the thymus, where they both expand and mature into immunologically competent cells (see Fig. 98 ). A variety of cytokines, including IL-2, IL-4, and lL-7, facilitate lymphopoiesis, whereas others such as transforming growth factor-/ may decelerate this process. T lymphocytes are probably the longest lived hematopoietic cell, as there is experimental evidence that the life span of some is more than 10 years. 

IL-3 stimulates the growth and differentiation of all hematopoietic lineages rather than self-renewal of primitive stem cells. Activated T cells secrete IL-3, but its effects are normally restricted to local cellular activation or to nearby sites where T cells migrate (paracrine action). Other general characteristics of IL-3 include ... 

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