What is phagocytosis in biology

Phagocytosis, or “cell eating”, is the process by which a cell engulfs a particle and digests it. The word phagocytosis comes from the Greek phago-, meaning “devouring”, and -cyte, meaning “cell”. Cells in the immune systems of organisms use phagocytosis to devour bodily intruders such as bacteria, and they also engulf and get rid of cell debris. Some single-celled organisms like amoebas use phagocytosis in order to eat and acquire nutrients.

The function of phagocytosis is to ingest solid particles into the cell. Phagocytosis is a type of endocytosis, which is when cells ingest molecules via active transport as opposed to molecules passively diffusing through a cell membrane. Only certain small molecules can pass through the cell membrane easily; larger ones have to go through special channels in the cell or be ingested via endocytosis. Other types of endocytosis include pinocytosis, also called “cell drinking”, and receptor-mediated endocytosis, which is when molecules bind to specific receptors on the cell membrane that causes the cell to engulf them.

Phagocytosis is different from pinocytosis because phagocytosis involves the ingestion of solid particles while pinocytosis is the ingestion of liquid droplets. Phagocytosis is also used by cells to take in much larger particles than those that are ingested through pinocytosis. Some single-celled protists, such as amoebae, use phagocytosis to ingest food particles; it is literally how they eat food. Since their entire body consists of one cell, they can ingest food particles through engulfing them, and then digest these particles by connecting with a lysosome. In pinocytosis, the particles that are engulfed do not need to be broken down by a lysosome because they are so small, and instead the vesicle empties its contents directly into the cell.

The cell that will perform phagocytosis is activated. This can be a phagocyte, which is a cell in the immune system that performs phagocytosis, or an organism such as an amoeba, which behaves in a similar way to phagocytes when it carries out phagocytosis. In the case of immune cells, activation occurs when the cells are near bacterial cells or parts of bacterial cells. Receptors on the surface of the cells bind to these molecules and cause the cells to respond.

In the immune system, chemotaxis may occur. Chemotaxis is the movement of phagocytes toward a concentration of molecules. Immune cells pick up chemical signals and migrate toward invading bacteria or damaged cells.

The cell attaches to the particle that it will ingest. Attachment is necessary for ingestion to occur. Some bacteria can resist attachment, making it harder for them to be taken into the cell and destroyed.

The cell ingests the particle, and the particle is enclosed in a vesicle (a sphere of cell membrane with fluid in it) called a phagosome. The phagosome transports the particle into the cell.

A lysosome fuses with the phagosome and the particle is digested. Lysosomes are vesicles that contain hydrolytic enzymes that break down molecules. A phagosome fused with a lysosome is called a phagolysosome.

Cellular waste, such as broken down molecules that the cell cannot reuse, is discharged from the cell by the process of exocytosis. Exocytosis is the opposite of endocytosis; it is when cellular waste products travel in vesicles to the surface of the cell membrane and are released, thereby exiting the cell.

Phagocytes are found throughout the human body as white blood cells in the blood. One liter of blood contains approximately six billion of them! Many different types of white blood cells are phagocytes, including macrophages, neutrophils, dendritic cells, and mast cells. White blood cells are known as “professional” phagocytes because their role in the body is to find and engulf invading bacteria. “Non-professional” phagocytes include other types of cells like epithelial cells, endothelial cells, and fibroblasts. These cells sometimes perform phagocytosis, but it is not their primary function.

As mentioned earlier in the article, amoebae perform phagocytosis in order to consume food particles. Amoebae engulf particles by surrounding them with pseudopods, which are temporary armlike projections of the cell that are filled with cytoplasm. Ciliates are another type of organisms that use phagocytosis to eat. Ciliates are protozoans that are found in water, and they eat bacteria and algae. Both amoebae and ciliates are protists, organisms that have eukaryotic cells but are not animals, plants, or fungi.

• Protist – An organism with eukaryotic cells that is not an animal, plant, or fungus; they are grouped together for convenience but not all are closely related.
• Endocytosis – The process by which cells ingest molecules through active transport as opposed to passive diffusion.
• Pinocytosis – The process by which a cell engulfs liquid droplets containing small particles.
• Phagocyte – An immune cell that gets rid of foreign material by ingesting it through phagocytosis.

1. What is the function of phagocytosis?
A. To destroy invading bacteria
B. To get rid of cell debris
C. To uptake nutrients
D. All of the above

D is correct. All of these are functions of phagocytosis, although they are performed by different cells. Choice A is performed by phagocytes in the immune system, B can be performed by phagocytes or other cells like endothelial cells, and choice C describes certain protists.

2. What is endocytosis?
A. When cells ingest particles through engulfing them
B. When cells ingest liquid droplets containing small particles
C. When cells expel waste products by bringing vacuoles to the surface of the cell membrane
D. When cells break down particles with hydrolytic enzymes

A is correct. Endocytosis includes phagocytosis, pinocytosis, and receptor-mediated endocytosis, among other processes. Choice B specifically describes pinocytosis only, choice C is exocytosis, and choice D is digestion in lysosomes (which is a step in endocytosis).

3. Which of the following does NOT enter a cell through phagocytosis?
A. Bacteria
B. Oxygen
C. Damaged cells or parts of cells
D. Algae

B is correct. Oxygen is a small molecule and it can easily diffuse through the cell membrane, which is important because animal cells need oxygen to survive and carry out cell processes. Larger molecules or even whole organisms like bacteria and algae are taken into a cell via phagocytosis. They are too large to simply pass through the cell membrane unaided.

Process by which a cell uses its plasma membrane to engulf a large particle

Phagocytosis (from Ancient Greek φαγεῖν (phagein) 'to eat', and κύτος (kytos) 'cell') is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is called a phagocyte.

In a multicellular organism's immune system, phagocytosis is a major mechanism used to remove pathogens and cell debris. The ingested material is then digested in the phagosome. Bacteria, dead tissue cells, and small mineral particles are all examples of objects that may be phagocytized. Some protozoa use phagocytosis as means to obtain nutrients.

History

Phagocytosis was first noted by Canadian physician William Osler (1876),[1] and later studied and named by Élie Metchnikoff (1880, 1883).[2]

In immune system

Main article: Phagocyte

Phagocytosis is one main mechanisms of the innate immune defense. It is one of the first processes responding to infection, and is also one of the initiating branches of an adaptive immune response. Although most cells are capable of phagocytosis, some cell types perform it as part of their main function. These are called 'professional phagocytes.' Phagocytosis is old in evolutionary terms, being present even in invertebrates.[3]

Professional phagocytic cells

Neutrophils, macrophages, monocytes, dendritic cells, osteoclasts and eosinophils can be classified as professional phagocytes.[2] The first three have the greatest role in immune response to most infections.[3]

The role of neutrophils is patrolling the bloodstream and rapid migration to the tissues in large numbers only in case of infection.[3] There they have direct microbicidal effect by phagocytosis. After ingestion, neutrophils are efficient in intracellular killing of pathogens. Neutrophils phagocytose mainly via the Fcγ receptors and complement receptors 1 and 3. The microbicidal effect of neutrophils is due to a large repertoire of molecules present in pre-formed granules. Enzymes and other molecules prepared in these granules are proteases, such as collagenase, gelatinase or serine proteases, myeloperoxidase, lactoferrin and antibiotic proteins. Degranulation of these into the phagosome, accompanied by high reactive oxygen species production (oxidative burst) is highly microbicidal.[4]

Monocytes, and the macrophages that mature from them, leave blood circulation to migrate through tissues. There they are resident cells and form a resting barrier.[3] Macrophages initiate phagocytosis by mannose receptors, scavenger receptors, Fcγ receptors and complement receptors 1, 3 and 4. Macrophages are long-lived and can continue phagocytosis by forming new lysosomes.[3][5]

Dendritic cells also reside in tissues and ingest pathogens by phagocytosis. Their role is not killing or clearance of microbes, but rather breaking them down for antigen presentation to the cells of the adaptive immune system.[3]

Initiating receptors

Receptors for phagocytosis can be divided into two categories by recognised molecules. The first, opsonic receptors, are dependent on opsonins.[6] Among these are receptors that recognise the Fc part of bound IgG antibodies, deposited complement or receptors, that recognise other opsonins of cell or plasma origin. Non-opsonic receptors include lectin-type receptors, Dectin receptor, or scavenger receptors. Some phagocytic pathways require a second signal from pattern recognition receptors (PRRs) activated by attachment to pathogen-associated molecular patterns (PAMPS), which leads to NF-κB activation.[2]

Fcγ receptors

Fcγ receptors recognise IgG coated targets. The main recognised part is the Fc fragment. The molecule of the receptor contain an intracellular ITAM domain or associates with an ITAM-containing adaptor molecule. ITAM domains transduce the signal from the surface of the phagocyte to the nucleus. For example, activating receptors of human macrophages are FcγRI, FcγRIIA, and FcγRIII.[5] Fcγ receptor mediated phagocytosis includes formation of protrusions of the cell called a 'phagocytic cup' and activates an oxidative burst in neutrophils.[4]

Complement receptors

These receptors recognise targets coated in C3b, C4b and C3bi from plasma complement. The extracellular domain of the receptors contains a lectin-like complement-binding domain. Recognition by complement receptors is not enough to cause internalisation without additional signals. In macrophages, the CR1, CR3 and CR4 are responsible for recognition of targets. Complement coated targets are internalised by 'sinking' into the phagocyte membrane, without any protrusions.[5]

Mannose receptors

Mannose and other pathogen-associated sugars, such as fucose, are recognised by the mannose receptor. Eight lectin-like domains form the extracellular part of the receptor. The ingestion mediated by the mannose receptor is distinct in molecular mechanisms from Fcγ receptor or complement receptor mediated phagocytosis.[5]

Phagosome

Engulfment of material is facilitated by the actin-myosin contractile system. The phagosome is the organelle formed by phagocytosis of material. It then moves toward the centrosome of the phagocyte and is fused with lysosomes, forming a phagolysosome and leading to degradation. Progressively, the phagolysosome is acidified, activating degradative enzymes.[2][7]

Degradation can be oxygen-dependent or oxygen-independent.

• Oxygen-dependent degradation depends on NADPH and the production of reactive oxygen species. Hydrogen peroxide and myeloperoxidase activate a halogenating system, which leads to the creation of hypochlorite and the destruction of bacteria.[8]
• Oxygen-independent degradation depends on the release of granules, containing enzymes such as lysozymes, and cationic proteins such as defensins. Other antimicrobial peptides are present in these granules, including lactoferrin, which sequesters iron to provide unfavourable growth conditions for bacteria. Other enzymes like hyaluronidase, lipase, collagenase, elastase, ribonuclease, deoxyribonuclease also play an important role in preventing the spread of infection and degradation of essential microbial biomolecules leading to cell death.[4][5]

Leukocytes generate hydrogen cyanide during phagocytosis, and can kill bacteria, fungi, and other pathogens by generating several other toxic chemicals.[9][10][11]

Some bacteria, for example Treponema pallidum, Escheria coli and Staphylococcus aureus, are able to avoid phagocytosis by several mechanisms.

In apoptosis

Following apoptosis, the dying cells need to be taken up into the surrounding tissues by macrophages in a process called efferocytosis. One of the features of an apoptotic cell is the presentation of a variety of intracellular molecules on the cell surface, such as calreticulin, phosphatidylserine (from the inner layer of the plasma membrane), annexin A1, oxidised LDL and altered glycans.[12] These molecules are recognised by receptors on the cell surface of the macrophage such as the phosphatidylserine receptor or by soluble (free-floating) receptors such as thrombospondin 1, GAS6, and MFGE8, which themselves then bind to other receptors on the macrophage such as CD36 and alpha-v beta-3 integrin. Defects in apoptotic cell clearance is usually associated with impaired phagocytosis of macrophages. Accumulation of apoptotic cell remnants often causes autoimmune disorders; thus pharmacological potentiation of phagocytosis has a medical potential in treatment of certain forms of autoimmune disorders.[13][14][15][16]

In protists

In many protists, phagocytosis is used as a means of feeding, providing part or all of their nourishment. This is called phagotrophic nutrition, distinguished from osmotrophic nutrition which takes place by absorption.[citation needed]

• In some, such as amoeba, phagocytosis takes place by surrounding the target object with pseudopods, as in animal phagocytes. In humans, the amoebozoan Entamoeba histolytica can phagocytose red blood cells.
• Ciliates also engage in phagocytosis.[17] In ciliates there is a specialized groove or chamber in the cell where phagocytosis takes place, called the cytostome or mouth.

As in phagocytic immune cells, the resulting phagosome may be merged with lysosomes(food vacuoles) containing digestive enzymes, forming a phagolysosome. The food particles will then be digested, and the released nutrients are diffused or transported into the cytosol for use in other metabolic processes.[18]

Mixotrophy can involve phagotrophic nutrition and phototrophic nutrition.[19]

See also

• Active transport
• Antigen presentation
• Antigen-presenting cell
• Emperipolesis
• Endosymbionts in protists
• Paracytophagy
• Phagoptosis
• Pinocytosis
• Residual body
• Cell wall

References

1. ^ Ambrose, Charles T. (2006). "The Osler slide, a demonstration of phagocytosis from 1876: Reports of phagocytosis before Metchnikoff's 1880 paper". Cellular Immunology. 240 (1): 1–4. doi:10.1016/j.cellimm.2006.05.008. PMID 16876776.
2. ^ a b c d Gordon, Siamon (March 2016). "Phagocytosis: An Immunobiologic Process". Immunity. 44 (3): 463–475. doi:10.1016/j.immuni.2016.02.026. PMID 26982354.
3. ^ a b c d e f M.), Murphy, Kenneth (Kenneth (2012). Janeway's immunobiology. Travers, Paul, 1956-, Walport, Mark., Janeway, Charles. (8th ed.). New York: Garland Science. ISBN 9780815342434. OCLC 733935898.
4. ^ a b c Witko-Sarsat, Véronique; Rieu, Philippe; Descamps-Latscha, Béatrice; Lesavre, Philippe; Halbwachs-Mecarelli, Lise (May 2000). "Neutrophils: Molecules, Functions and Pathophysiological Aspects". Laboratory Investigation. 80 (5): 617–653. doi:10.1038/labinvest.3780067. ISSN 0023-6837. PMID 10830774.
5. ^ a b c d e Aderem, Alan; Underhill, David M. (April 1999). "Mechanisms of Phagocytosis in Macrophages". Annual Review of Immunology. 17 (1): 593–623. doi:10.1146/annurev.immunol.17.1.593. ISSN 0732-0582. PMID 10358769.
6. ^ The Immune System, Peter Parham, Garland Science, 2nd edition
7. ^ Flannagan, Ronald S.; Jaumouillé, Valentin; Grinstein, Sergio (2012-02-28). "The Cell Biology of Phagocytosis". Annual Review of Pathology: Mechanisms of Disease. 7 (1): 61–98. doi:10.1146/annurev-pathol-011811-132445. ISSN 1553-4006. PMID 21910624.
8. ^ Hemilä, Harri (1992). "Vitamin C and the common cold". British Journal of Nutrition. 67 (1): 3–16. doi:10.1079/bjn19920004. PMID 1547201.
9. ^ Borowitz JL, Gunasekar PG, Isom GE (12 Sep 1997). "Hydrogen cyanide generation by mu-opiate receptor activation: possible neuromodulatory role of endogenous cyanide". Brain Research. 768 (1–2): 294–300. doi:10.1016/S0006-8993(97)00659-8. PMID 9369328. S2CID 12277593.
10. ^ Stelmaszyńska, T (1985). "Formation of HCN by human phagocytosing neutrophils--1. Chlorination of Staphylococcus epidermidis as a source of HCN". Int J Biochem. 17 (3): 373–9. doi:10.1016/0020-711x(85)90213-7. PMID 2989021.
11. ^ Zgliczyński, Jan Maciej; Stelmaszyńska, Teresa (1988). The Respiratory Burst and its Physiological Significance. pp. 315–347. doi:10.1007/978-1-4684-5496-3_15. ISBN 978-1-4684-5498-7.
12. ^ Bilyy RO, Shkandina T, Tomin A, Muñoz LE, Franz S, Antonyuk V, Kit YY, Zirngibl M, Fürnrohr BG, Janko C, Lauber K, Schiller M, Schett G, Stoika RS, Herrmann M (January 2012). "Macrophages discriminate glycosylation patterns of apoptotic cell-derived microparticles". The Journal of Biological Chemistry. 287 (1): 496–503. doi:10.1074/jbc.M111.273144. PMC 3249103. PMID 22074924.
13. ^ Mukundan L, Odegaard JI, Morel CR, Heredia JE, Mwangi JW, Ricardo-Gonzalez RR, Goh YP, Eagle AR, Dunn SE, Awakuni JU, Nguyen KD, Steinman L, Michie SA, Chawla A (November 2009). "PPAR-delta senses and orchestrates clearance of apoptotic cells to promote tolerance". Nature Medicine. 15 (11): 1266–72. doi:10.1038/nm.2048. PMC 2783696. PMID 19838202.
14. ^ Roszer, T; Menéndez-Gutiérrez, MP; Lefterova, MI; Alameda, D; Núñez, V; Lazar, MA; Fischer, T; Ricote, M (Jan 1, 2011). "Autoimmune kidney disease and impaired engulfment of apoptotic cells in mice with macrophage peroxisome proliferator-activated receptor gamma or retinoid X receptor alpha deficiency". Journal of Immunology. 186 (1): 621–31. doi:10.4049/jimmunol.1002230. PMC 4038038. PMID 21135166.
15. ^ Kruse, K; Janko, C; Urbonaviciute, V; Mierke, CT; Winkler, TH; Voll, RE; Schett, G; Muñoz, LE; Herrmann, M (September 2010). "Inefficient clearance of dying cells in patients with SLE: anti-dsDNA autoantibodies, MFG-E8, HMGB-1 and other players". Apoptosis. 15 (9): 1098–113. doi:10.1007/s10495-010-0478-8. PMID 20198437. S2CID 12729066.
16. ^ Han, CZ; Ravichandran, KS (Dec 23, 2011). "Metabolic connections during apoptotic cell engulfment". Cell. 147 (7): 1442–5. doi:10.1016/j.cell.2011.12.006. PMC 3254670. PMID 22196723.
17. ^ Grønlien HK, Berg T, Løvlie AM (July 2002). "In the polymorphic ciliate Tetrahymena vorax, the non-selective phagocytosis seen in microstomes changes to a highly selective process in macrostomes". The Journal of Experimental Biology. 205 (Pt 14): 2089–97. doi:10.1242/jeb.205.14.2089. PMID 12089212.
18. ^ Montagnes, Djs; Barbosa, Ab; Boenigk, J; Davidson, K; Jürgens, K; Macek, M; Parry, Jd; Roberts, Ec; imek, K (2008-09-18). "Selective feeding behaviour of key free-living protists: avenues for continued study". Aquatic Microbial Ecology. 53: 83–98. doi:10.3354/ame01229. ISSN 0948-3055.
19. ^ Stibor H, Sommer U (April 2003). "Mixotrophy of a photosynthetic flagellate viewed from an optimal foraging perspective". Protist. 154 (1): 91–8. doi:10.1078/143446103764928512. PMID 12812372.

External links

Wikisource has the text of the 1911 Encyclopædia Britannica article "Phagocytosis".

• 
  Media related to Phagocytosis at Wikimedia Commons
• Phagocytosis at the US National Library of Medicine Medical Subject Headings (MeSH)

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