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Phagocytosis (Pct)

Phagocytosis (Pct): Overview and Significance

Phagocytosis (Pct), derived from the Ancient Greek words “phagein” (to eat) and “kytos” (cell), is a fundamental biological process where a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), such as microbes, dead cells, and debris[1][4]. The term refers to the process by which certain living cells called phagocytes ingest or engulf other cells or particles[6]. This process gives rise to an internal compartment called the phagosome[4].

Purpose of Phagocytosis

Phagocytosis serves several critical functions:

• Immune Defense: It is a major mechanism used by the immune system to remove pathogens and cell debris[4].
• Tissue Homeostasis: It plays a vital role in eliminating apoptotic cells and maintaining tissue balance[5].
• Nutrient Acquisition: Some protozoa use phagocytosis as a means to obtain nutrients, referred to as phagotrophy[4].
• Tissue Development and Remodeling: It is required for the constant substitution of senescent cells and clearing old cell debris gathered inside tissues[1].

The Process of Phagocytosis

The phenomenon of phagocytosis occurs through these main steps[1][5]:

1. Recognition of the Target Particle: The cell membrane of phagocytes has receptors that bind to foreign substances. The receptor is usually the receptor of the target that binds to antibodies which have marked a pathogen[1]. A range of receptors are involved in the phagocytic process. Some receptors attach to particular ligands and need co-receptors to conduct phagocytosis efficiently. In contrast, a different set of phagocytic receptors are fully functional in conducting both binding and engulfment of the target substances[1].
2. Signaling to Activate the Internalization Machinery: These receptors activate the signal transduction pathways, initiating F-actin polymerization at the site of the bound substances[1].
3. Engulfment/Phagosome Formation: Endocytosis causes the cytoplasm to engulf the bound pathogen[1]. The plasma membrane of a cell is directed by cytoskeletal filaments to form pseudopodia (fake arms) that act to engulf a particle and bring it inside the cell[7]. F-actin remodeling helps in the engulfment of the bound substances and is critical in the phagocytic process[1].
4. Phagolysosome Maturation: The engulfed particle is placed in a phagosome, which in turn associates with the lysosomes in the cell[1]. 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[1]. Chemical reactions occur, damaging and destroying the particle via oxidative stress[1].
5. Elimination: Through exocytosis, the remnants are propelled out[1].

Cells Involved in Phagocytosis

The cells that perform phagocytosis are called phagocytes[4]. Some of these are macrophages, dendritic cells, and neutrophils[1]. Such cells express different receptors which activate signals in the cells leading to phagocytosis[1]. Neutrophils and macrophages are essential for immunity[3]. Phagocytosis takes place in any tissue, usually in the bloodstream, the intestinal space, alveoli of the respiratory system, etc[1].

Clinical Significance

Phagocytosis is a vital aspect of the immune system, and its dysfunction can lead to various diseases[3][5]. By destroying pathogens, the immune system stops the spread of infection or pathogens in the body[3]. This process is used in clearing old cell debris that gets gathered inside tissues, causing damage through the phenomenon of secondary necrosis[1].

Conclusion

Phagocytosis is an essential process in immunity, tissue homeostasis, and nutrient acquisition. It involves a series of steps, including recognition, engulfment, and digestion of particles by specialized cells called phagocytes. Understanding phagocytosis is crucial for comprehending the mechanisms of the immune system and developing strategies to combat various diseases.

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References

1. Diagram of Phagocytosis – BYJU’S. Link
2. Phagocytosis Definition, Types & Steps – Lesson – Study.com. Link
3. Discover the meaning of phagocytosis, its origin and FAQS – BYJU’S. Link
4. Phagocytosis – Wikipedia. Link
5. Phagocytosis: A Fundamental Process in Immunity – PMC. Link
6. Phagocytosis | Definition, Process, & Examples – Britannica. Link
7. Phagocytosis – StatPearls – NCBI Bookshelf. Link
8. Phagocytosis: MedlinePlus Medical Encyclopedia Image. Link