The thymus is where T cells of the adaptive immune system mature. Thymocytes are created by hematopoietic stem cells in the bone marrow and migrate to the thymus, where they undergo a process of change and selection to become T cells capable of deploying an immune response against pathogens and harmful cells, but not against healthy cells. Unfortunately, the active thymic tissue that guides this process atrophies with age in process known as involution. By the age of 50 most people have little functional thymic tissue left, and as a consequence the production of new T cells is greatly reduced. This loss of reinforcements is a major contribution to the age-related decline of the adaptive immune system. Without replacements, it becomes a cluttered mess of exhausted, senescent, and misconfigured cells. The overall number of T cells stays much the same, but their quality and behavior declines precipitously.
Given this, regrowth of active thymic tissue is an important goal for the rejuvenation research community. Several approaches have been demonstrated to achieve this goal in older mice, resulting in a restored production of T cells
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