NK Cells: What They Are And What Functions They Have In The Human Body
Surely you have heard of “killer cells” at some point. 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 their other functions are beyond destroying “malignant” or abnormal cells.
In addition, we will also explain its relationship with the KIR receptors and the MHC (major histocompatibility complex).
NK cells, also called Natural Killer (NK) 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 in our immune system, along with T and B lymphocytes.
But… What differentiates NK cells from T and B lymphocytes? The fact that it belongs to the innate immune system, and is 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 cancerous or tumor cells. On the other hand, they also regulate the different immune responses of the immune system; Furthermore, 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 They act mainly thanks to a family of receptors called “Immunoglobuline-like receptors” (KIR), which allow them to respond to alterations present in infected or cancerous cells, whose 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 on NK cells, they can recognize infected and cancerous cells very specifically ; 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 act? 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 rupture 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 it carried out stop.
But how do NK cells recognize infected or cancerous cells? The studies carried out show that they probably do this through two mechanisms: either they detect these cells by recognizing a type of material they contain, called glycocalyx, and which is altered, or through the loss, in these cancer cells, major histocompatibility complex (MHC or MHC) class I.
Specifically, NK cells have a series of receptors on their membrane that allow detecting the presence of MHC class 1 in altered or abnormal cells ; In healthy cells, these receptors are inhibited (which is why NK cells are able to distinguish them, thanks to this recognition system, which is very effective).
Let us remember 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 the antigens to the T lymphocytes, which allows different processes involved in the body’s immune response to be activated.
How are NK cells activated?
We have seen, broadly speaking, how NK cells work. But how are they activated by an infected or cancerous cell?
They do it 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 is worth mentioning 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, their antigens are presented to the infected cell (on its surface), and antibodies attached 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 this type of cells.
It could be said that NK cells constitute the body’s first line of innate defense which responds to infections and tumor transformations that occur in cancer cells.
Furthermore, these cells have a high discriminative power, since they can differentiate between cells infected by a virus and cells affected by a tumor.
Let us 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 the body’s own cells and “invading” or foreign cells.
