Thymic involution is the process by which the thymus, a small organ located behind the sternum, gradually decreases in size and function as a person ages. This process typically begins during puberty and continues throughout adulthood. The thymus is crucial for the development of T cells, a type of white blood cell essential for the immune system. As the thymus involutes, it is replaced by fatty tissue, leading to a decline in the production of new T cells. This reduction can impact the immune response, making older individuals more susceptible to infections and diseases. Despite this, the body retains a pool of memory T cells that can respond to previously encountered pathogens.