Every day, inside our body, there is a war going on. Microscopic invaders of one kind or another try to make a meal of us, and our immune system fights back, seeking out the invaders and destroying them. One of our body’s most important foot-soldiers in this war is the T cell, a type of white blood cell with receptors that can recognize foreign substances. Like all white blood cells, T cells originate in the bone marrow, but then they migrate to an organ called the thymus (hence the “T” in “T cell”), where they evolve into specialized immune system warriors. Mature T cells, which leave the thymus and circulate around the body, come in different types. One type, the cytotoxic T cell, specializes in attacking and killing cells of the body that are infected by viruses, bacteria, or cancer.