NK cells: what they are and what functions they have in the human body

You have surely heard of “killer cells” before. Also known as Natural Killer or, for short, NK cellsThese types of cells are lymphocytes of the innate immune system and are responsible for neutralizing cells infected with viruses or bacteria, as well as cancer cells.

Its proper functioning is essential to prevent many types of cancer, as well as other pathologies. In this article, we will explain in detail what they are, how they work and how they are activated, and what are some of their functions beyond destroying “malignant” or abnormal cells.

In addition, we will also explain its relationship with KIR receptors and MHC (main histocompatibility complex).

    NK cells: definition and general characteristics

    NK cells, also called Natural Killer (NK) cells, are natural lymphocytes of the innate immune system., The main function is to protect our body. This type of cell represents one of three groups of lymphocytes in our immune system, along with T and B lymphocytes.

    But … What differentiates NK cells from T and B lymphocytes? Belonging to the innate immune system and part of the first line of defense against a very wide range of pathogens.

    NK cells destroy two types of cells: infected cells (by viruses, bacteria, etc.) and cancer or tumor cells. On the other hand, they also regulate the different immune responses of the immune system; in addition, they are involved in the rejection of bone marrow transplants, in the processes of autoimmunity and in the maintenance of pregnancies.

    As we will see, NK cells they act mainly through a family of receptors called “Immunoglobulin-like receptors” (KIR), Which enable them to respond to alterations present in infected or carcinogenic cells, HLA class I molecules (major histocompatibility complex) are altered. Later, we will discuss what this histocompatibility complex consists of.

    KIR receivers

    Thanks to the KIR receptors of NK cells, these they can very specifically recognize infected and cancerous cells; this is possible thanks to the signals they receive through numerous receptors on malignant cells, which eventually trigger their cytotoxicity, as well as the secretion of chemokines and cytokines.

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      How do NK cells work? What they do is destroy the aforementioned cells by attacking their plasma membrane, Which causes a process called cytolysis (or cytolysis), and which consists in the rupture of the cell by the decomposition of its cell membrane; in this process, moreover, the cell loses its genetic material and the vital processes that it has carried out stop.

      But how do NK cells recognize infected or cancerous cells? Studies show that they probably do this by two mechanisms: either they detect these cells by recognizing a type of material in them, called glycocalyx, which is altered, or by loss, in these cancer cells, of the major complex of histocompatibility (MHC or MHC) class I.

      Specifically, NK cells have a number of receptors in their membrane which they make it possible to detect the presence of MHC class 1 in altered or abnormal cells; in healthy cells, these receptors are inhibited (this is why NK cells are able to distinguish them, thanks to this recognition system, which is very efficient).

      Major histocompatibility complex

      Remember that MHC or MHC is a family of genes located on a chromosome, Specifically chromosome 6 (in humans).

      Its function is to encode leukocyte antigens (or histocompatibility antigens); these antigens, in turn, have the task of presenting the antigens to the T lymphocytes, which makes it possible to activate various processes involved in the body’s immune response.

      How are NK cells activated?

      We have seen, in general, how NK cells work. But how do they activate in the face of an infected or cancerous cell?

      they do thanks to a group of signaling proteins called interferons (IFNs); interferons are produced by host cells when a virus, bacteria, parasite or tumor cell infects them, through a feedback process.

      Beyond interferons, NK cells they are also activated by other types of substances, such as interleukins-2, Which are cytokines (a type of protein) synthesized in T lymphocytes. It should be mentioned here that NK cells activated by interleukin 2 in the laboratory are called “LAK cells”.

      On the other hand, NK cells have on their surface a number of specific receptors for immunoglobulin G (a type of antibody); when these cells find a cell infected with a virus, their antigens are present in the infected cell (on its surface) and the antibodies bound to the NK cell bind to the infected cell.

      the functions

      NK cells are of great importance for the health and proper functioning of our body, because prevent infected cells from continuing to live and perpetuate themselves. This is because of its ability to recognize and kill these types of cells.

      One could say that NK cells constitute the body’s innate first line of defense, Which responds to infections and tumor transformations that occur in cancer cells.

      In addition, these cells have a high discriminating power, as they can differentiate between cells infected with a virus and cells affected by a tumor.

      Remember that these last cells are those which have undergone various malignant transformations in their structure. On the other hand, NK cells are also able to differentiate between cells in the body itself and “invasive” or foreign cells.

      Bibliographical references:

      • Abbas, A., Lichtman, A. and Pillai, S. (2008). Cellular and molecular immunology. 6th edition. Section IV: Effective mechanisms of immune responses; Cytokines.
      • Gil, RA (2005). Regulation of activation-induced cell death in T lymphocytes. Role of alpha diacylglycerol kinase. Doctoral thesis. Institute of Biology and Molecular Genetics University of Valladolid-CSIC.
      • Goldberg, Anna Carla; Rizzo, Luiz Vicente. (2015). Structure and function of the MHC: presentation of the antigen Part 1. Einstein (São Paulo) (in English) (San Pablo, Brazil: Scielo), 13 (1): 153-156.
      • Mace ME, Dongre P, Hsu HT, SinhaP, James AM, Mann SS, Forbes LR, Watkin LB, Orange JS (2014). Biological steps of cells and checkpoints to access NK cell cytotoxicity. Immunology and Cell Biology (review), 92: 245-255.
      • Schleinitz N, March ME, Long EO. (2008). Acquisition of activating receptors at the level of immunological synapses of inhibitory NK cells. PLoS ONE, 3 (9): e3278.
      • Sepúlveda, C. and Pont, J. (2000). Natural killer cells and the innate immune system in infectious disease. Tower. med. Chile, 128 (12).

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