The thymus is a specialized organ of the immune system that is located in the upper chest, just behind the sternum (breastbone). It is most active during childhood and adolescence and plays a key role in the development and maturation of T-cells, which are a type of white blood cell that plays a central role in the immune response.
The thymus produces and secretes a variety of hormones, such as thymosin, which are involved in the development and differentiation of T-cells. As we age, the thymus gradually shrinks and becomes less active, which may contribute to the decline in immune function that occurs with aging.
Further Reading:
Immune System Structure, Function, & Health Tips