The thymus is the major site of maturation of T cells. This function of the thymus was first suspected because of immunologic deficiencies associated with the lack of a thymus. The congenital absence of the thymus, as occurs in the DiGeorge syndrome in humans or in the nude mouse strain, is characterized by low numbers of mature T cells in the circulation and peripheral lymphoid tissues and severe deficiencies in T cell-mediated immunity (see Chapter 20). If the thymus is removed from a neonatal mouse, this animal fails to develop mature T cells. The thymic anlage develops from the endoderm of the third pharyngeal pouch and the underlying neural crest-derived mesenchyme and is subsequently populated by bone marrow-derived precursors. The thymus involutes with age and is virtually undetectable in postpubertal humans, resulting in a somewhat reduced output of mature T cells. However, maturation of T cells continues throughout adult life, as indicated by the successful reconstitution of the immune system in adult recipients of bone marrow transplants. It may be that the remnant of the involuted thymus is adequate for some T cell maturation. Because memory T cells have a long life span
Stage of maturation
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