Phagocytes include macrophages and neutrophils. Both cell types ingest foreign materials by phagocytosis (cell-eating). Phagocytosis is one form of endocytosis (bringing materials into the cell) which involves the intake of large partciles, such as pahtogens.
Chemicals are released at sites of injury or inflammation that attract neutrophils and monocytes that are in the vasculature. This is called chemotaxis. To get to the site of injury, which is within the tissues and outside the blood vessels, the cells must adhere to the blood vessel wall and squeeze through to enter the tissues.
Neutrophils arrive first at sites of injury. Monocytes are stored in the spleen. They circulate in the blood stream and enter tissues to become macrophages. Macrophages engulf foreign materials and worn-out neutrophils.
Phagocytes recognize invaders (pathogens) as foreign. The pathogen binds to the cell surface and is engulfed by the phagocyte, forming a membrane-bound phagosome. The phagosome fuses with a lysosome within the cell. The lysosome contains enzymes that digest the foreign material, making it inactive. For example, digestion of an engulfed bacterium renders it unable to divide and produce more bacteria. Undigested materials accumulate in the phagocyte, eventually killing it.
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