Two people born on the exact same day can age in totally different ways ‪—‬ one in relative health and the other with chronic disease. Now, scientists have found a key difference in the immune systems of people who are more prone to age-related conditions, and it could help explain why some people remain healthier longer. In young people, the immune system contains many naive T cells, a type of white blood cell. Over time, exposure to pathogens and vaccinations transforms some of these T cells into memory cells, which can respond quickly to defend the body against germs. These memory cells make different types of characteristic proteins, called granzymes, which kill infected cells. They come...