The thymus is an organ at the top of the chest just beneath the top of the breastbone. The thymus is made of two lobes and is an irregular shaped lumpy gland. At birth the thymus is quite large, but its size reduces with age and by the time an individual reaches puberty is very small.

The thymus is one of two primary lymphoid organs (PLOs) in the human body, the other being the bone marrow. T-lymphocytes are "born" in the bone marrow, but it is in that thymus that these cells are selected and mature.

T-lymphocytes that show a strong response to self tissue are removed from the circulation, a process known as central tolerance. Remaining T-lymphocytes mature and then leave the thymus to contribute to cell-mediated immunity of the adaptive immune system.



Soluble Mediators