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Natural killer cells

Most lymphocytes are primarily involved in the specific adaptive immune response, and thus will be discussed in the following chapter. An exception is the natural killer cells (NK cells) ; these mononuclear lymphocytes use nonspecific mechanisms to recognize and destroy cells that are abnormal in some way. Cancer cells and cells infected with viruses are two examples of cellular abnormalities that are targeted by NK cells. Recognition of such cells involves a complex process of identifying inhibitory and activating molecular markers on the surface of the target cell. Molecular markers that make up the major histocompatibility complex (MHC) are expressed by healthy cells as an indication of “self.” This will be covered in more detail in next chapter. NK cells are able to recognize normal MHC markers on the surface of healthy cells, and these MHC markers serve as an inhibitory signal preventing NK cell activation. However, cancer cells and virus-infected cells actively diminish or eliminate expression of MHC markers on their surface. When these MHC markers are diminished or absent, the NK cell interprets this as an abnormality and a cell in distress. This is one part of the NK cell activation process ( [link] ). NK cells are also activated by binding to activating molecular molecules on the target cell. These activating molecular molecules include “altered self” or “nonself” molecules. When a NK cell recognizes a decrease in inhibitory normal MHC molecules and an increase in activating molecules on the surface of a cell, the NK cell will be activated to eliminate the cell in distress.

NK cells have both inhibitory and activating receptors. Normal cells have signals on their MHC molecules that bind to the inhibitory receptors; so the NK cell does not kill them. Cells that are infected with virus have ligands that bind to the activating receptor; this causes the NK cell to kill them.
Natural killer (NK) cells are inhibited by the presence of the major histocompatibility cell (MHC) receptor on healthy cells. Cancer cells and virus-infected cells have reduced expression of MHC and increased expression of activating molecules. When a NK cell recognizes decreased MHC and increased activating molecules, it will kill the abnormal cell.

Once a cell has been recognized as a target, the NK cell can use several different mechanisms to kill its target. For example, it may express cytotoxic membrane proteins and cytokines that stimulate the target cell to undergo apoptosis , or controlled cell suicide. NK cells may also use perforin-mediated cytotoxicity to induce apoptosis in target cells. This mechanism relies on two toxins released from granules in the cytoplasm of the NK cell: perforin , a protein that creates pores in the target cell, and granzymes , proteases that enter through the pores into the target cell’s cytoplasm, where they trigger a cascade of protein activation that leads to apoptosis. The NK cell binds to the abnormal target cell, releases its destructive payload, and detaches from the target cell. While the target cell undergoes apoptosis, the NK cell synthesizes more perforin and proteases to use on its next target.

NK cells contain these toxic compounds in granules in their cytoplasm. When stained, the granules are azurophilic and can be visualized under a light microscope ( [link] ). Even though they have granules, NK cells are not considered granulocytes because their granules are far less numerous than those found in true granulocytes. Furthermore, NK cells have a different lineage than granulocytes, arising from lymphoid rather than myeloid stem cells ( [link] ).

Questions & Answers

intracellular vesicles are found in
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how food can be used as a substrate for the growth of microorganisms
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What is different between eukaryotes and prokcaryotic
Abia Reply
main difference is that eukaryotic cells possess membrane bound organelles
Iqra
prokaryotes are primitive organisms that doesn't contain membrane bound nucleus or any orgenelle while in eukaryotes membrane bound orgenelles and nucleus is present
Yashi
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jane Reply
its mean body structure, function, and systems
gazi
study of internal structure of living things
Falere
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isir
anotomy is the actual study of body internally and externally Which include how itis made. for what and what is need of this
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Jeewraj
Linnaeus used Aristotle's criteria for dividing living organisms into kingdoms. Plants are immobile and insensitive, while animals are mobile and sensitive. the creatures he saw through the microscope were mobile, so he attributed them to animals.
Lad
name the different types of media use in lab to detect the micro organisms
Shehzadi Reply
The different types of media used in the lab to detect the microorganisms is known as cell culture..
Tean
solid media or broth is used to grow and detect microorganisms
Pohor
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Baraka
the study of large living organisms
Pratibha
the study of organisms which are micro in range
Vency
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process for sterilization
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is a machine used on the process of sterilisation
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Jennifer
Robert Hooke
Martha
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Jennifer Reply
Antonie van Leeuwenhoek
Satarupa
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icrobiology is the scientific study of these microorganisms. Microorganisms are those organisms that are too small to see with the naked eye and include things like bacteria, fungi, and viruses
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to differentiate between gram positive and gram negative bacty
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Rana
arif bhai gram staining in which we are given different dyes to the bacterial cell wall and cytoplasm and nucleus.e.g giemsa stain
Rana
Gram staining uses crystal violet to stain cell walls iodine as a mordant and a fuchsin or safranin counterstain to mark all bacteria
Umer
Gram staining uses crystal violet to stain cell walls iodine as a mordant and a fuchsin or safranin counterstain to mark all bacteria
Umer
Gram staining uses crystal violet to stain cell walls iodine as a mordant and a fuchsin or safranin counterstain to mark all bacteria
Umer
Omar baba crystal violet iodine 70% alcohol fuchicine or safranine after every step washing must be done
Rana
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Cristy
*Fuchsin stain
Cristy
*Safranin
Cristy
thanks
Ganesh
difference between epidermophyton trichophyton and microsporum
Ankita Reply
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Iqra
epidermophyton type of fungi causes superficial and cutaneous mycoses trichophyton is also fungi type including parasitic varieties causes dermatophytosis microsporum is also type of fungi causes dermatophytosis.
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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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