Even as an infant is learning to distinguish herself from the rest of the world, a key part of her immune system is working toward the same goal, albeit in a very different manner.
From before birth through the early years of life, young immune system cells in the thymus gland are receiving an education in their native surroundings. Many of these cells, called thymocytes, will eventually mature into T cells, which will protect the body from foreign invaders like bacteria and viruses and from rogue actors like cancer cells. For now, though, they’re going through a ruthless sorting process that will determine whether they live or die.
