Phagocytosis

Phagocytosis, often called cell eating, is a type of endocytosis in which a cell takes in particles big enough to see under a light microscope, such as microbes, debris or whole dying cells. Single-celled organisms like amoebas use it to feed. In animals it is the work of specialized white blood cells called phagocytes, chiefly macrophages, neutrophils and dendritic cells.

It happens in steps. The phagocyte first recognizes its target through surface receptors, which may bind molecules typical of microbes, such as pieces of bacterial cell wall, or antibodies and complement proteins that coat the target, a tagging process called opsonization. The cell then remodels its actin cytoskeleton to push out arm-like pseudopods that flow around the particle and close over it, trapping it in a membrane sac called a phagosome. The phagosome fuses with lysosomes, and inside the combined compartment, acid, digestive enzymes and reactive oxygen compounds kill and break down the contents.

Phagocytosis is a core part of innate immunity and also clears away the many cells that die naturally each day. Macrophages and dendritic cells display fragments of what they have eaten to T cells, linking innate and adaptive defenses. Some pathogens, including the bacterium that causes tuberculosis, survive by resisting digestion inside phagocytes.