The thymus gland

The thymus gland is in the chest, between the lungs and behind the breastbone (sternum). It is just in front of, and above, the heart.

The thymus makes white blood cells called T lymphocytes (also called T cells). These are an important part of the body's immune system, which helps us to fight infection. The thymus produces all our T cells before we become teenagers. It gradually becomes less active and eventually gets smaller and is replaced by fat tissue. The thymus also produces a hormone called thymosin that helps make and develop T cells.