Phagocytosis

Phagocytosis is the ingestion of microorganisms or particulate matter by a cell. Phagocytosis is performed by phagocytes, certain types of white blood cells or their derivatives. Among the granulocytes, neutrophils are the most important phagocytes. Enlarged monocytes become wandering macrophages and fixed macrophages. Fixed macrophages are located in selected tissues and are part of the mononuclear phagocytic system. Granulocytes predominate during the early stages of infection, whereas monocytes predominate as the infection subsides. Chemotaxis is the process by which phagocytes are attracted to microorganisms. The phagocyte then adheres to the microbial cells; adherence may be facilitated by opsonization—coating the microbe with serum proteins. Pseudopods of phagocytes engulf the microorganism and enclose it in a phagocytic vesicle to complete ingestion. Many phagocytized microorganisms are killed by lysosomal enzymes and oxidizing agents. Some microbes are not killed by phagocytes and can even reproduce in phagocytes. Evasion mechanisms include M protein, capsules, leukocidins, membrane attack complexes, and prevention of phagolysosome formation.

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