The thymus is a pinkish-gray organ that sits just below the sternum. It functions as a school for your immune system’s T cells, creating them from blood stem cells, then educating them to seek and destroy infections such as viruses, bacteria and fungi as well as cancer cells. But the thymus naturally shrinks throughout life, and that largely corresponds with the body’s declining ability to fight off pathogens.
Researchers have been trying to find a way to stall or even reverse the thymus’s shrinking. Richard Boyd, an immunologist at Monash University in Melbourne, Australia, has focused on puberty, a time when the organ undergoes a dramatic reduction in size. Because testosterone, estrogen and other sex hormones ramp up during adolescence, Boyd wondered if these same hormones might somehow prompt the thymus to “involute”—to shrivel.
To test the idea, he castrated adult male mice by surgically removing their testes, halting production of testosterone. Sure enough, within four weeks, the animals’ thymi grew nearly as large and capable as those in younger animals. “It was a lightbulb moment,” Boyd says.
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