Phagocytosis and its process


  • The process of ingestion of bacteria or other material by phagocytes and amoeboid protozoans is called phagocytosis.
  • Macrophages and neutrophils show a high capacity for the physical engulfment of particles such as microorganisms or microbial fragments from their immediate extracellular environment.
  • There are other various cells which show this process as well.
  • For engulfing of the microorganisms, they have to pass through a chain of molecular processes. They are as follows:

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A) Chemo-taxis of the phagocyte toward the microorganism

  • Firstly, phagocytes are attracted towards the microorganisms.
  • This attraction is due to the various reasons like
  1. Signals arising from the microorganism itself (e.g. formylmethionyl peptide).
  2. Signals arising from complement proteins (e.g. C3a and C5a) which generates as a part of the activation of the alternative complement pathway.
  3. Signals due to release of inflammatory factors (e.g. leukotrines) secreted by other leucocyte cells situated at the site of action.

B) Adherence of microorganism to the surface of the phagocyte

  • After attraction, the microorganisms adhere to the phagocyte by various processes like:
  1. Adhesion through lectin receptors present on the surface of the phagocyte. These receptors interact with carbohydrate moieties on the surface of the microorganism.
  2. Adhesion through complement C3b receptors present on the surface of phagocyte interacting with C3b molecules that have opsonized the surface of a microorganism.
  3. Adhesion through Fc receptors which interact with the Fc domain of antibodies that have opsonized the surface of a microorganism.

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C) Membrane activation

  • It involves various processes like:
  1. Actin-myosin contractile network of phagocyte activates to extend pseudopodia around the attached microorganisms.
  2. This activation also leads to the generation of a ‘respiratory burst’ by a phagocyte.
  3. It involves an increase in the activity of the phagocyte membrane NADPH oxidase.
  4. This enzyme converts molecular oxygen into bactericidal reactive oxygen species like superoxide anion (O2), hydrogen peroxide (H2O2), and in particular hydroxyl radicals (-OH) and halogenated oxygen metabolites (HOCl).

D) Formation of phagosome

  • The enclosure of phagocytosed material, initially with a membranous vesicle is called phagosome.
  • Cationic proteins such as defensins and reactive oxygen species begin microbial membrane degradation.
  • Phagosome gets fused with lysosome within minutes resulting in the formation of phagolysosome.
  • The contents of phagolysosome are at an acidic pH of 5 which is optimal for the continuous active breakdown of the microbial structural components.

E) Expelling of waste materials

  • The unwanted waste materials after digestion of microorganisms are expelled out.




Phagocytosis and its process