Where do "T" cells get their name? Until the 1960s, the thymus, a small butterfly-shaped organ inside the upper chest, had no known function. It went from being described as the “seat of the soul” in ancient times to a useless vestigial organ in the early 20th century. The thymus was known to house immune cells, but experiments in adult mice showed that removing it led to no defects in immune response. Its purpose remained elusive. But in 1961, Jacques Miller, then a PhD student at The Institute of Cancer Research in London, led a series of groundbreaking experiments. He discovered that removing the thymus in mice right after birth (instead of adulthood) rendered them quickly susceptible to infection. Miller ran further tests by transplanting skin from other mice strains and rats. Foreign skin should’ve triggered an immune response, but it didn’t. Miller had just proved that the thymus played a critical role in the immune system. He concluded that the thymus, at least at the time of birth, was the source of important immune cells that then migrated to other sites, where they remained and fought pathogens. These came to be known as “thymus-derived” cells, or T cells. Later research would show that T cells actually originate in the bone marrow as stem cells, but then travel to the thymus to mature. Cool fact: Miller is widely credited for being the last person to discover the function of a major human organ, and also the only living person to have done so. Image credits: nature.com/articles/s41418-0… health.harvard.edu/blog/got-…