Around Christmas of 1882, while peering through a microscope at starfish larvae in which he had inserted tiny thorns, Russian zoologist Elie Metchnikoff had a brilliant insight: what if the mobile cells he saw gathering around the thorns were nothing but a healing force in action? Metchnikoff's daring theory of immunity--that voracious cells he called phagocytes formed the first line of defense against invading bacteria--would eventually earn the...