- 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:
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
- Signals arising from the microorganism itself (e.g. formylmethionyl peptide).
- Signals arising from complement proteins (e.g. C3a and C5a) which generates as a part of the activation of the alternative complement pathway.
- 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:
- Adhesion through lectin receptors present on the surface of the phagocyte. These receptors interact with carbohydrate moieties on the surface of the microorganism.
- Adhesion through complement C3b receptors present on the surface of phagocyte interacting with C3b molecules that have opsonized the surface of a microorganism.
- 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:
- Actin-myosin contractile network of phagocyte activates to extend pseudopodia around the attached microorganisms.
- This activation also leads to the generation of a ‘respiratory burst’ by a phagocyte.
- It involves an increase in the activity of the phagocyte membrane NADPH oxidase.
- 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