Phagocytosis: A Review

Abstract

Phagocytosis is the ingestion of solid particulate matter by cells, chiefly leukocytes. The engulfment is followed by sequestration of the ingested material within phagocytic vacuoles into which the contents of leukocytic granules are poured.

Phagocytosis is augmented by the frequency of encounters between cells and particles. Such encounters are abetted by chemotactic factors which purposefully direct phagocytes along concentration gradients toward the agents to be ingested. Chemotaxins exerting direct effects on cells are called cytotaxins, and those acting indirectly are called cytotaxigens. The former are proteins or polypeptides; the latter are often particulate and may be proteins, polysaccharides, lipids, or smaller molecules. Chemotaxins are generated by microorganisms chiefly through activation of serum complement. Inactivators and inhibitors of cytotaxins also exist. It is suggested that eventually there is binding of chemotactic peptides with a stereospecific receptor on the surface of the phagocyte which induces cellular locomotion in the appropriate direction, either by sol-gel transformation or by the contraction and relaxation of microfilaments and microtubules.

Unless the cell recognizes the object to be ingested, phagocytosis will not follow. Recognition is related to surface phenomena, and opsonic proteins are implicated, including antibodies and components of the complement and properdin systems. They probably impart molecular configurations to the surface of the particle which facilitate recognition by receptors on the phagocyte. Recognition is the prelude to attachment and phagocytosis. Various divalent cations promote or inhibit ingestion of particles while other components of the cell surface, such as the glycoproteins responsible for the binding of lectins, may also be involved.

Adhesion of the particle to the cell surface is most frequently followed by its ingestion. The role of cations in overcoming electrostatic forces of repulsion is briefly discussed, and the morphological aspects of phagocytosis viewed by light, transmission, and scanning electron microscopy described. It appears that circumferential attachment is essential for ingestion to occur.

Metabolic changes in phagocytes during the processes of chemotaxis, ingestion, and digestion are reviewed, as are the antimicrobial mechanisms available to the cell. These microbicidal activities form two broad classes: those which are oxygen dependent and those which are independent. In the first class, some components are myeloperoxidase dependent and generate hydrogen peroxide, superoxide anion, and the related hydroperoxyl radical. Various cofactors are participants in the system, including halides such as chloride, iodide, and bromide. The microbicidal mechanisms involve an interaction of hydrogen peroxide with the iron of the heme groups of myeloperoxidase, producing a strongly oxidizing enzyme-substrate complex which forms lethal chemicals by oxidation of the available halides. Singlet oxygen may also be formed with chemiluminescence during phagocytosis. Other components of the oxygen-dependent class of antimicrobial systems are independent of myeloperoxidase, because hydrogen peroxide, superoxide anions, hydroxyls, and singlet oxygen may all be produced under aerobic conditions.

Oxygen-independent antimicrobial processes also form a second ancillary class of microbicidal activities in phagocytes. The principal agents of this system are acid, lysozyme, lactoferrin, and a group of cationic proteins. Generally, it appears that these various antimicrobial mechanisms more than adequately ensure the destruction of microorganisms.