Thymus

The thymus is a specialized organ of the immune system. Within the thymus, T-cells, critical to the adaptive immune system, mature. The thymus is composed of two identical lobes and is located anatomically in the anterior superior mediastinum, in front of the heart and behind the sternum.

The thymus provides an inductive environment for development of T-lymphocytes from hematopoietic progenitor cells. In addition, thymic stromal cells allow for the selection of a functional and self-tolerant T-cell repertoire. Therefore, one of the most important roles of the thymus is the induction of central tolerance.

The thymus is largest and most active during the neonatal and pre-adolescent periods. By the early teens, the thymus begins to atrophy and thymic stroma is mostly replaced by adipose (fat) tissue. Nevertheless, residual T lymphopoiesis continues throughout adult life.