Phagocytosis

2 Key excerpts on "Phagocytosis"

• 

  eBook - ePub

  Veterinary Immunology - E-Book

  • Ian R. Tizard(Author)
  • 2017(Publication Date)
  • Saunders

    (Publisher)

  Borrelia burgdorferi, where a single lamellipod may wrap itself several times around the organism. This is called coiling Phagocytosis.

  Destruction

  Neutrophils kill ingested bacteria through two distinct processes. One involves the generation of potent oxidants by a respiratory burst. The other involves release of lytic enzymes and antimicrobial peptides from intracellular granules (Box 5.1 ).

  Box 5.1 Autophagy

  Phagocytosis, as described in this chapter, involves the ingestion, killing, and digestion of extracellular particles such as invading bacteria. Cells can also destroy particles within the cytosol by autophagy (see Fig. 5.16 ). Autophagy is a form of cellular waste disposal. The structure to be destroyed, such as an intracellular microbe or a damaged cytoplasmic organelle, is first enclosed within a double membrane to form a cytosolic vesicle called an autophagosome. This then fuses with lysosomes, whose enzymes digest the contents of the autophagosome. Their macromolecules are then released back into the cytosol, where they can be recycled. Autophagy can be triggered by starvation to provide more amino acids for protein synthesis, but it can also be used to selectively remove organelles such as mitochondria, misfolded and aggregated proteins, and intracellular infectious agents. Thus TLR7 or FcγR signaling from phagosomes can initiate their targeting by the autophagy system, possibly by acting through the NOX complex. Autophagy plays important roles in elimination of intracellular pathogens, activation of intracellular pattern-recognition receptors, regulation of inflammasome activation, and intracellular antigen processing. Disorders of autophagy are associated with cancer, neurodegeneration, microbial infections, and aging.

  FIG. 5.16

  The process of autophagy, a method of cellular waste disposal. An autophagosome forms within the cytoplasm and encloses the organelle or microbe to be destroyed. Lysosomes fuse with the autophagosome to form an autolysosome. The contents are degraded and recycled. This is a way to get rid of old, damaged organelles as well as intracellular bacteria.

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  eBook - ePub

  Medical Immunology, 7th Edition

  • Gabriel Virella(Author)
  • 2019(Publication Date)
  • CRC Press

    (Publisher)

  Israel J. Med. Sci ., 18, 897–916, 1982.)

  Several recognition systems appear to be involved in the Phagocytosis step. The most important recognition systems are those that mediate the ingestion of opsonized particles. Two major types of receptors expressed by phagocytic cells are involved in this process:

  ● Fcγ receptors are predominantly involved in promoting ingestion of antibody-coated particles. Neutrophils express two types of Fcγ receptors, Fcγ RII and Fcγ RIII, both of which are involved in Phagocytosis. In experimental conditions, Fcα receptors may also be involved in Phagocytosis, but their efficiency seems to be much lower than that of Fcγ receptors.

  ● CR1 (C3b) receptor is also able to mediate Phagocytosis with high efficiency. This receptor is expressed by all phagocytic cells, including polymorphonuclear leukocytes, monocytes, and macrophages. The binding and ingestion of microorganisms through this receptor have been well established.

  Opsonization with both IgG antibodies and C3b seems associated with maximal efficiency in ingestion. However, opsonization is not an absolute requirement for ingestion by phagocytic cells. A variety of receptors may be involved in nonimmune Phagocytosis, as described in greater detail in Chapter 14 . These nonimmune mechanisms are particularly effective in promoting the ingestion of microorganisms with polysaccharide-rich outer layers. In addition, neutrophils are also able to ingest a variety of particulate matter, such as latex beads, silicone, asbestos fibers, etc., in the absence of opsonizing antibodies or complement.

  Intracellular killing

  Irrespectively of the nature of the receptors that may mediate it, ingestion is achieved through formation of pseudopodia that surround the particle or bacteria, and eventually fuse at the distal pole forming a phagosome. The cytoplasmic granules of the neutrophil (lysosomes) then fuse with the phagosomes, and their contents empty inside the phagosomes (degranulation). This degranulation process is very rapid and delivers a variety of antimicrobial substances to the phagosome:

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