A phagosome is the membrane-bound sac formed inside a phagocyte when it engulfs a bacterium, particle, or dead cell. The cell’s membrane wraps around the target and pinches off, internalising it into this vesicle, which then fuses with lysosomes full of digestive enzymes and antimicrobial chemicals to form a phagolysosome where the contents are destroyed. The process, phagocytosis, is the immune system’s core clearance mechanism, and the phagosome is its crucible. Some pathogens, notably Mycobacterium tuberculosis, have evolved to survive inside phagosomes by blocking fusion with lysosomes, turning the cell’s weapon into a shelter. Nutritional deficiencies that impair phagocyte function, particularly in protein, zinc, and vitamin A, weaken this defence at its most fundamental level, the intracellular killing chamber.