The thymus is a small gland located in the upper chest, 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. This reduction in size can affect the immune response, making older individuals more susceptible to infections. Despite its decline, the thymus remains important for immune function throughout life.