For decades, the thymus, a small organ tucked behind the breastbone, has been all but forgotten. Long thought to matter only in childhood and fade into irrelevance in adulthood, that assumption is now being challenged. We may be only as young as this part of the immune system we have ignored for too long.
Beginning in adolescence, the thymus does not simply shrink, it is steadily replaced by fat in a process called “involution”. By midlife, its production of new T cells has dropped sharply. In later decades, the thymus is small and largely inactive, and the immune system becomes…
