"However, these cells can be screened and destroyed. Young T-cells are sent to a "nursery" called a thymus, located above the heart. "The thymus is like a 'mini-me' of yourself," explains Punt. "It's a protective environment that allows the developing cells to browse a representation of your body." Ninety-five percent of these cells are killed before they can be released into the body; those that do "graduate" from the thymus will identify and destroy only pathogens-hopefully."