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What is phagocytosis : a definition of the term and useful vocabulary

It is clear that cells have adopted and developed several mechanisms that make it possible to internalize large solid particles as well as solutes and liquid substrates. In the current state of understanding, it is considered conventional to subdivide the process of phagocytosis on several specific activities of a living cell, namely: 1) pinocytosis, which represents a way to engulf bulks of liquid substrate and fluids; 2) phagocytosis itself is very similar to pinocytosis, with the only difference being that the phagosome contains solid particles; 3) receptor-mediated endocytosis, which provides the formation of phagolysosome involving specific uptake in a clathrin-dependant manner. Speaking of multicellular organisms, we can subdivide phagocytosis in compliance with the main types of targets, which are dying cells and, specifically, microbial pathogens alongside with other allogenic particles. Therefore, in this case, the main question, such as what is phagocytosis must be approached from a detached view of phagocytes’ functions that are, predominantly, control of immunological responses, eliminating inflammations, clearance of old and dying cells (which is connected with precise sensing and internalization) as well as maintenance of tissue homeostasis.

As opposed to writing a persuasive essay a well-written original research on a biological topic requires both a qualified coursework definition and thoroughly accumulated knowledge of the topic. Let us overlook a set of crucial terms and definitions that are widely accepted among modern immunology scientists. Hence, the core vocabulary includes the following items:

  • Apoptosis is an active form of cell death, which is also known under the colloquial name of ‘cellular suicide’. First of all, apoptosis is a programmed and strictly planned cell death that requires ATP molecules and is taking place step-by-step; one can without fail distinguish phases of the release of apoptotic bodies, irreversible chromatin condensation, nuclear fragmentation and membrane blistering. Various methods, such as analysis of phagocytosis kinetics, responses of phagocytes, cellular death kinetics are used to clarify what exactly an apoptotic cell is undergoing during its programmed death. Quite a lot of details of apoptosis still remain in severe need of elucidation.
  • Programmed cell death is a wide term that describes a series of molecular transformations consequential from the decision of a cell to die. Earlier, PCD was considered as a synonym of apoptosis; however, contemporary immunology marks out the latter as one of the forms of programmed cell death. To be more exact, today we know autophagy and necrosis along with apoptosis, and all these terms have their specific and appropriate application. It is essential to establish which form of PCD a cell tends to choose under different circumstances and what mechanisms allow it to switch from autophagy to necrosis or apoptosis.
  • Autophagy refers to a response that is triggered by extreme conditions and results in the delivery of cytoplasmic constituents to the lysosome. This partially happens with many cellular contents as well, as part of a regular cytoplasmic turnover. The process of autophagy comprises several steps, namely: sequestration of some cytoplasmic organelles, transportation of the sequestered content followed by lysosomal fusion, total biological destruction of the debris parts and turning the degradation products in the cell metabolic cycle. The data available on this phenomenon are a great asset for a student wanting to comprehend what is phagocytosis.
  • Primary necrosis has a significantly different appearance from all said phenomenon. While autophagy can be used by the cell in order to benefit to the supply of energy or boost ATP to survive during a critical period, primary necrosis is what one may call an ‘accidental cell death’, meaning that a cell does not feature a program to execute such a procedure. Necrosis happens mainly when the membrane cannot sustain its integrity and succumbs to spontaneous lysis. This is followed by spillage of cytoplasm and other variegated consequences of the induction of necrosis, which are always crowned by the death of the cell.
  • Secondary necrosis is what happens when living cells programmed to elimination by apoptosis fail to run the procedure. If such mistaken cells do not become a substrate for phagocytes they usually undergo the process of secondary necrosis. Eventually, all cells are going to be lysed; however, consequences of postponed or delayed lysis differ greatly from those of primary necrosis. The issue of signalization between cells submitted to secondary necrosis and other cells that undergo normal apoptosis and necrosis is one of big interest for contemporary immunology.
  • Phagocytosis index is a value that is defined by the amount of apoptotic cells attached to 100 phagocytes; minutely speaking, this term is a synonym of ‘interaction index’. Moreover, this concept is assembled from many other immunological terms, such as internalization, adhesion and tethering and reinforced by a well-balanced quantitative parameter.

Discussing specific questions: what are professional and nonprofessional phagocytes

Generally speaking, a cell that is not undergoing any said death-related phenomenon can be competently considered viable. Today, electronic microscopy is thought to be the standard for determination of cell viability. Undoubtedly, EM has long become a routine procedure alongside with many even simpler and cheaper experimental methods. Other methods employing dyeing and chemical cell impregnation are also possible only after cell fixation and death (for instance, dyeing with trypan blue or propidium iodide). Nevertheless, there is no shortage of specialists who do a routine examination and careful monitoring of cellular processes using light microscopy alone or any other approaches that enable researchers to conduct the experiment without killing a cell culture. Regrettably, these techniques are often insensible to the early stages of PCD and, therefore, cannot be counted as universal or widely applicable.

It is highly important to remember that advantages of cell lines depend greatly on the degree of their invariance. Unquestionably, the precision of a laboratory experiment has little to do with religious studies, which means it is not possible for a researcher to keep faith in the purity of cell lines. One way or another, all cell lines must be assessed for accumulated mutations before enabling them into any laboratory procedure. This concerns lines of phagocytes in the first places. The terms non-professional and professional phagocytes are connected with a specific function of a cell; that is, the latter refers to dendritic cells and macrophages, while the former consists of cells that are primarily localized near the cell that dies. Depending on what site and current level of activity the given cell characterizes of it may get involved in different kinds of cell-to-cell interactions and perform variously under specific conditions.

First and foremost, a student should bear in mind before asking about how to write a coursework that the nature of apoptotic cells engulfed by professional phagocytes still remains an unresolved issue. Some researchers believe that those cells are, indubitably, a source of tolerogenic antigens. Moreover, this topic becomes especially controversial when it is a matter of tissue-specific autoantigens. Clearance of dying cells is the issue of the biggest concern for both neighboring accidental phagocytes and professional killer cells. However, it has become widely accepted that cells undergoing apoptosis could be a top-priority source of stimulatory autoantigens in case clearance is reduced, delayed or, for some reason, stopped. The term efferocytosis referring to the phagocytosis of apoptotic cells brings in even more subjects for a long-standing discussion. It seems, the characterization of the corresponding receptors as well as “calm” determinants on either an apoptotic cell or professional phagocyte will be practically indispensable for stable and substantive molecular progress.

What is phagocytosis : a short historical summary and modern state of researches of the phenomenon

The phenomenon of phagocytosis was first discovered by Russian scientist Ilya Ilyich Mechnikov and is a derivation from Greek words meaning “devouring cells”. On equal terms with Paul Ehrlich, I.I. Mechnikov was awarded the Nobel Prize in Physiology and Medicine in 1908. This highly important biological process was observed in transparent starfish larvae; mostly, the definition of phagocytosis given by contemporary science is still within the primary understanding – that is, we define phagocytosis as the process of engulfment and destruction of solid particles, which size exceeds 0.5 µm. As long as a student intends to compose a 500 word essay or a terse overview related to the phenomenon of phagocytosis, it will be useful for them to read and analyze original works by Mechnikov. Being significantly ahead of his time, the Russian immunologist managed to identify more or less fully a defensive function of phagocytes as well as provide a unique description of apoptotic cell clearance by macrophage and overlook the involvement of phagocytes in the degeneration of host cells by the example of tadpoles. Of course, the previous decades of scientific development have contributed a large amount of experimental data and now we can witness profound progress in understanding the molecular mechanisms and functional implementation of phagocytosis.

While simple and amoebae-like organisms use the phenomenon of phagocytosis to supply their cells (or a single cell) with nutrients and energy, larger and more complex organisms have specialized functions of their particular cells up to the level where they are known as professional phagocytes. Apart from those killer cells, there are also other – for instance, ladder epithelial cells – that can successfully perform clearance of junk cells or cellular debris under certain conditions. Simultaneously, professional killer cells, such as dendritic cells, neutrophils, and macrophages are irreplaceable for tissue defense and immune surveillance with the object to eliminate bacteria, fungi, infectious pathogens and other allogenic agents. Accordingly, the question what is phagocytosis starts, primarily, at the cellular level, albeit it is strongly related to molecular and genetic specificity. Basically, contemporary immunology is interested in recognizing receptors and specificity of phagocytosis above all. According to the most recent data, the direct interaction of an allogenic agent with the phagocyte is possible due to PRR (pattern recognition receptors) and specificity of this process has roots in pathogen’s conserved structural motifs. At the same time, another group of highly specific allogenic proteins that are known under the name of opsonins promotes the discovery of the opsonic receptors. For instance, the Fcγ receptor, which is connected to the immunoglobulin antibody, is responsible for phagocyte activation as well as complement receptors that were found on dendritic cells and macrophages.

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