Wanting to know the secrets of her longevity, scientists have examined her blood and other tissues, and found some interesting things. For one, they found that about two-thirds of her white blood cells had been made by only two stem cells, as reported by New Scientist. (Stem cells create white blood cells, which are responsible for fighting off infection, and other functions.) Typically, about 1,000 stem cells actively make these blood cells. That finding implies that, predictably, stem cells die out throughout the years, and that there is indeed a limit to human life.