Definition
The process by which microbes, particles or fragments of dead cells are internalized and engulfed, typically by particular membrane receptors, is called phagocytosis.
Example – A diagram illustrating phagocytosis in Amoeba
It is a process of ingestion of particles. This cellular process to ingest and eliminate particles includes microbes and apoptotic cells. The phenomenon is required for tissue balance; a range of cell types can conduct the process. Having said that, only special cells called phagocytes conduct the process with extreme efficiency. Some of these are macrophages, dendritic cells and neutrophils. Such cells express different receptors which activate signals in the cells leading to phagocytosis.
Process of Phagocytosis
The phenomenon of phagocytosis occurs through these steps –
- Particle is recognized
- Particle is ingested
- Formation of early phagosome
- Formation of late phagosome
- Formation of phagolysosome
Phagocytosis is used to engulf large solid particles and is always driven by F-actin polymerization. Additionally, particular uptake of large particles is initiated by the binding and identification of ligands on the target substances by receptors on the surface of phagocytic cells.
Engulfment of Particles
A range of receptors are involved in the phagocytic process, but a few of these receptors take part in attaching to the particular ligands and need co-receptors to conduct phagocytosis efficiently.
Contrastingly, a different set of phagocytic receptors are fully functional in conducting both binding and engulfment of the target substances. The latter receptors activate the signal transduction pathways, initiating F-actin polymerization at the site of the bound substances and results in particle engulfment.
F-actin remodeling helps in the engulfment of the bound substances and is critical in the phagocytic process. After internalization, the ingested substances are enclosed in a vacuole called phagosome that unites with the endosomes and lysosomes, which culminates in a highly acidic compartment called mature phagolysosome.
Working process of Phagocytosis
Phagocytes engulf debris or pathogens through the process of phagocytosis. It takes place in any tissue, usually in the bloodstream, the intestinal space, alveoli of the respiratory system etc.
There may be variations to the process, however, generally, the process occurs as follows –
- The cell membrane of the phagocytes has receptors which bind to foreign substances. The receptor is usually the receptor of the target that binds to antibodies which have marked a pathogen
- Endocytosis causes cytoplasm to engulf the bound pathogen
- The engulfed particle is placed in phagosome which in turn associates with the lysosomes in the cell
- Chemical reactions occur damaging and destroying the particle via oxidative stress
- Through exocytosis, the remnants are propelled out
Significance of Phagocytosis
- Large multicellular organisms require constant substitute of senescent cells which are formed at the time of development and tissue repair
- Studies show a functional requirement for phagocytosis in tissue development and remodeling
- Phagocytosis is used in clearing old cell debris that gets gathered inside tissues causing damage through the phenomenon of secondary necrosis
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