This episode covers one half of the 1908 Nobel Prize in Physiology or Medicine, which was awarded to Ilya Ilyich Mechnikov. The Nobel Assembly at the Karolinska Institute chose to give Mechnikov the award “in recognition of [his] work on immunity”. Topics include a description of phagocytosis, Mechnikov’s experiments with phagocytic cells of the immune system, and the remarkable ways cells like macrophages and neutrophils attack harmful bacteria.