What Are Phagocytes?

Phagocytes are cells that have phagocytosis. From the morphological point of view, phagocytic cells are divided into two categories: one is small phagocytic cells; the other is large phagocytic cells ^[1] . Small phagocytic cells are neutrophils in peripheral blood. Macrophages are monocytes in the blood and macrophages in various organs and tissues. Both constitute a mononuclear phagocyte system.

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The play of phagocytosis involves the directed migration, recognition, phagocytosis and killing of phagocytic cells.

(1) Contact of phagocytes with pathogenic organisms: Phagocytic cells aggregate and migrate to the invasion site of pathogenic organisms under the action of chemokines (cytokines, activated complement fragments, certain bacterial cell components and products), Pathogens meet, and their surface receptors (mannose receptors, lipopolysaccharide receptors) recognize pathogens and their corresponding ligands and bind to them. It can also contact the corresponding pathogen through the LGF receptor and C3b receptor.

(2) Swallowing pathogenic organisms: Phagocytic cells extend to the pseudo-foot to surround larger particles of bacteria and other foreign matter, and they are taken into the cells to form phagosomes surrounded by a layer of cell membrane, that is, phagocytosis. For smaller substances such as viruses, the phagocytic membrane of the attachment site is invaded to form phagocytic vesicles, that is, cell drinks.

(3) Killing pathogenic organisms: After the formation of phagosomes, the lysosomes in the phagocytic cells approach the phagosomes and fuse with them to form the phagolysosomes. Peroxidase can kill pathogenic organisms, and proteases, polysaccharides, nucleases, lipases, etc. break it down, and finally the indigestible residues are excreted outside the phagocytosis cells

The consequences of phagocytosis vary with the type, virulence, and immune status of the pathogen that the phagocytes engulf.

(1) Complete phagocytosis: refers to phagocytic cells that can both swallow and kill and digest pathogenic organisms. For example, pyogenic cocci usually die within 5 to 10 minutes and are digested and decomposed within 30 to 60 minutes.

(2) Incomplete phagocytosis: refers to phagocytic cells that can only swallow pathogenic organisms but cannot kill and digest them. Incomplete phagocytosis can protect the ingested pathogens from phagocytic cells from the effects of non-specific antimicrobial molecules, specific antibodies or antibiotics in body fluids. Some pathogens can even reproduce in phagocytic cells, leading to death of phagocytes, or the phagocytic cells that travel with them can spread to other parts of the body through lymph fluid or blood, causing widespread infection. Incomplete phagocytosis is more common in phagocytic cells phagocytosis of intracellular parasitic microorganisms such as Mycobacterium tuberculosis and viruses. In addition, during phagocytosis, various enzymes released by lysosomes can destroy nearby normal tissue cells while sterilizing and digesting foreign bodies, causing immune pathological damage to tissues. ^[2]