How does the size of the thymus change from adolescence to old age?

The thymus is a key organ in the lymphatic and immune systems, responsible for the maturation of T lymphocytes or T cells, which are crucial for immune response. During adolescence, the thymus reaches its peak development as it is most active in producing T cells during this phase of growth and development. However, as a person ages, particularly beyond puberty, the thymus gradually undergoes a process called involution, where it decreases in size and functional activity.

This reduction in size is primarily due to the replacement of thymic tissue with fatty tissue, resulting in a decrease in the number of functional T cells produced. Consequently, by old age, the thymus can be significantly smaller, contributing to a decline in immune function known as immunosenescence. Understanding this process highlights the dynamic changes in immune function over the lifespan, particularly how the development of the immune system is linked to the physical state of the thymus.