<P> Phagocytosis is the process of taking in particles such as bacteria, parasites, dead host cells, and cellular and foreign debris by a cell . It involves a chain of molecular processes . Phagocytosis occurs after the foreign body, a bacterial cell, for example, has bound to molecules called "receptors" that are on the surface of the phagocyte . The phagocyte then stretches itself around the bacterium and engulfs it . Phagocytosis of bacteria by human neutrophils takes on average nine minutes . Once inside this phagocyte, the bacterium is trapped in a compartment called a phagosome . Within one minute the phagosome merges with either a lysosome or a granule to form a phagolysosome . The bacterium is then subjected to an overwhelming array of killing mechanisms and is dead a few minutes later . Dendritic cells and macrophages are not so fast, and phagocytosis can take many hours in these cells . Macrophages are slow and untidy eaters; they engulf huge quantities of material and frequently release some undigested back into the tissues . This debris serves as a signal to recruit more phagocytes from the blood . Phagocytes have voracious appetites; scientists have even fed macrophages with iron filings and then used a small magnet to separate them from other cells . </P> <P> A phagocyte has many types of receptors on its surface that are used to bind material . They include opsonin receptors, scavenger receptors, and Toll - like receptors . Opsonin receptors increase the phagocytosis of bacteria that have been coated with immunoglobulin G (IgG) antibodies or with complement . "Complement" is the name given to a complex series of protein molecules found in the blood that destroy cells or mark them for destruction . Scavenger receptors bind to a large range of molecules on the surface of bacterial cells, and Toll - like receptors--so called because of their similarity to well - studied receptors in fruit flies that are encoded by the Toll gene--bind to more specific molecules . Binding to Toll - like receptors increases phagocytosis and causes the phagocyte to release a group of hormones that cause inflammation . </P> <P> The killing of microbes is a critical function of phagocytes that is performed either within the phagocyte (intracellular killing) or outside of the phagocyte (extracellular killing). </P> <P> When a phagocyte ingests bacteria (or any material), its oxygen consumption increases . The increase in oxygen consumption, called a respiratory burst, produces reactive oxygen - containing molecules that are anti-microbial . The oxygen compounds are toxic to both the invader and the cell itself, so they are kept in compartments inside the cell . This method of killing invading microbes by using the reactive oxygen - containing molecules is referred to as oxygen - dependent intracellular killing, of which there are two types . </P>

Explain why some white blood cells are called phagocytes