Phagocytosis

Phagocytosis is a type of endocytosis where cells engulf large particles like bacteria, viruses, or cellular debris. The process begins when cell surface receptors bind to a particle, triggering the cell to extend projections called pseudopods that surround and envelop the material into a membrane-bound vesicle called a phagosome.

Specialized immune cells carry out phagocytosis in mammals, including macrophages, neutrophils, and dendritic cells. These cells use phagocytosis to eliminate unwelcome pathogens like viruses and bacteria, as well as infected host cells and cellular debris from damaged tissue.

After the phagosome forms, it moves through the cytoplasm and fuses with a lysosome to create a phagolysosome. Inside this structure, degradative enzymes digest the engulfed material. Eventually, the phagolysosome forms a waste-containing residual body that is released from the cell.

Phagocytes recognize particles through surface receptors that bind to the particle when contact occurs. Sometimes this is a chance encounter, but other times the phagocyte follows chemical signals to the particle in a process called chemotaxis. Different phagocytes use distinct receptors that may be general or specific.

Pseudopods are projections of cytoplasm that the phagocyte extends around a bound particle. These projections continue to surround the particle until it is completely enveloped and pinched off into the cytoplasm, forming the phagosome that contains the engulfed material.

Phagocytosis is energy-requiring because it involves active cellular processes including receptor binding, pseudopod extension, membrane fusion, and vesicle movement through the cytoplasm. These dynamic steps require ATP to power the molecular machinery that recognizes, surrounds, and processes engulfed particles.

Phagocytosis is one of three major types of endocytosis and is specialized for engulfing large objects like cells, bacteria, and viruses. Unlike receptor-mediated endocytosis, which targets specific molecules, phagocytosis uses general or specific surface receptors to recognize and engulf large particles through pseudopod extension.