Phagocytic Cells

Phagocytes are the white blood cells that protect the body by ingesting (phagocytosing) harmful foreign particles, bacteria and dead or dying cells. They are essential for fighting infections, and for subsequent immunity. Phagocytes are important throughout the animal kingdom, and are highly developed in vertebrates. One liter of human blood contains about six billion phagocytes. Their name comes from the Greek phagein, 'to eat or devour', and kutos, 'hollow vessel'.

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. Then the phagocyte 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 imprisoned bacterium is then submitted to a formidable battery 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 but 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 will eat almost anything; scientists have fed macrophages with iron filings and then used a small magnet to separate them from other cells in a mixture.

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 complement or IgG antibodies. Complement is the name given to a complex series of protein molecules found in the blood that destroy or mark cells 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