The thymus is a small gland located in the upper chest, just behind the sternum. It plays a crucial role in the immune system, particularly during childhood. The thymus is responsible for the maturation of T cells, a type of white blood cell that helps the body fight infections and diseases. As a person ages, the thymus gradually shrinks and becomes less active, a process known as thymic involution. Despite its reduced size, the thymus remains important for maintaining immune function. It produces hormones, such as thymosin, that aid in the development and differentiation of T cells.