Phagocytosis is the cellular process by which phagocytes engulf and destroy bacteria, foreign particles, and dead cells. The phagocyte’s membrane extends around the target, internalising it into a vesicle called a phagosome, which then fuses with lysosomes packed with digestive enzymes and reactive chemicals that dismantle the invader. It is the immune system’s primary clearance mechanism, operating constantly in tissues throughout the body, and it bridges innate and adaptive immunity when dendritic cells present fragments of digested pathogens to lymphocytes. Nutritional status profoundly affects the process: deficiencies in protein, zinc, iron, or vitamin A blunt phagocytic killing, which is why malnourished individuals succumb to infections that well-nourished bodies clear routinely. Phagocytosis was first described by Élie Metchnikoff, earning him a Nobel Prize.