Nk Cells: What Are They And What Functions Do They Have In The Human Body

What are NK cells? Let’s see the characteristics of these and how they protect the body.

NK cells

Surely you’ve ever heard of “killer cells.” Also known as Natural Killer or, for short, NK cells, these types of cells are lymphocytes of the innate immune system, and are responsible for neutralizing cells infected by viruses or bacteria, as well as cancer cells.

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

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

NK cells: definition and general characteristics

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

But … what differentiates NK cells from T and B lymphocytes? The fact of belonging to the innate immune system, and of being part of the first line of defense against a very wide range of pathogens.

What NK cells do is destroy two types of cells: infected cells (by viruses, bacteria …) 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 autoimmunity processes and in the maintenance of pregnancies.

As we will see, NK cells act mainly thanks to a family of receptors called “Immunoglobuline-like receptors” (KIR), which allow them to respond to alterations present in infected or cancer cells, whose HLA class I molecules (major histocompatibility complex ) are altered. Later on, we will discuss what this histocompatibility complex consists of.

KIR receivers

Thanks to the KIR receptors on NK cells, they can recognize infected and cancer cells in a very specific way ; This is possible thanks to the signals they receive through many of the malignant cell receptors, which end up triggering their cytotoxicity, as well as the secretion of chemokines and cytokines.

Functioning

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), which consists of the breaking of the cell through the decomposition of its cell membrane; In this process, in addition, the cell loses its genetic material and the vital processes that it was carrying out are stopped.

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

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

Major histocompatibility complex

Recall that CMH 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 mission of presenting antigens to T lymphocytes, which allows different processes involved in the body’s immune response to be activated.

How are NK cells activated?

We have seen, roughly, how NK cells work. But how are they activated by an infected or cancerous cell?

They do so thanks to a group of signaling proteins called interferons (IFNs) ; interferons are produced by host cells when infected by a virus, bacterium, parasite or tumor cell, through a feedback process.

Beyond interferons, NK cells 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 interleukins- 2 in the laboratory are called “LAK cells.”

On the other hand, NK cells have on their surface a series of specific receptors for immunoglobulin G (a type of antibody); When these cells encounter a cell infected by a virus, its antigens are presented on the infected cell (on its surface), and antibodies bound to the NK cell bind to the infected cell.

Features

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

It could be said that NK cells are the body’s innate first line of defense, responding to infections and tumor transformations that occur in cancer cells.

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

Remember that these last cells are those that have undergone different malignant transformations in their structure. On the other hand, NK cells are also capable of differentiating between cells from the body itself and “invasive” or foreign cells.

Bibliographic references:

  • Abbas, A., Lichtman, A. and Pillai, S. (2008). Cellular and Molecular Immunology. 6th Edition. Section IV: effector mechanisms of immune responses; Cytokines
  • Gil, RA (2005). Regulation of activation-induced cell death in T lymphocytes. Role of Diacylglycerol Kinase alfa. Doctoral Thesis. Institute of Molecular Biology and Genetics University of Valladolid-CSIC.
  • Goldberg, Anna Carla; Rizzo, Luiz Vicente. (2015). MHC structure and function – antigen presentation. Part 1. Einstein (São Paulo) (in English) (São Paulo, Brazil: Scielo), 13 (1): 153-156.
  • Mace ME, Dongre P, Hsu HT, SinhaP, James AM, Mann SS, Forbes LR, Watkin LB, Orange JS (2014). Cell biological steps and checkpoints in accessing NK cell cytotoxicity. Immunology and Cell Biology (Review), 92: 245-255.
  • Schleinitz N, March ME, Long EO. (2008). Recruitment of Activation Receptors at Inhibitory NK Cell Immune Synapses. PLoS ONE, 3 (9): e3278.
  • Sepúlveda, C. and Puente, J. (2000). Natural killer cells and the innate immune system in infectious disease. Rev. medic. Chile, 128 (12).

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