The thymus is a specialized lymphoid organ of the immune system. The thymus is in front of the heart and behind the breastbone.

In the thymus, T cells or T lymphocytes mature. T cells are critical to the adaptive immune system, where the body adapts specifically to foreign invaders.

The thymus is where T cells develop from hematopoietic (blood-forming) cells. In addition, this is where the T cells get tolerant to cells of the body.