What Are Natural Killer Cells?

Humans have natural killer cells that control several types of tumors and microbial infections by limiting the damage. Read more to know how essential it is.

What Are Natural Killer Cells?

Natural Killer (NK) Cells are lymphocytes that share a common progenitor with T and B cells and belong to the same family. However, NK cells are categorized as group I Innate Lymphocytes (ILCs), cells of the innate immune system, and react fast to a wide range of pathogenic stimuli.

NK cells are recognized for destroying virally contaminated cells and spotting and managing early cancer indications. Specialized NK cells are present in the placenta and may be crucial throughout pregnancy in addition to defending against illness, according to the British Society for Immunology.

Compared to cytotoxic T cells, which require antigen-presenting cells for priming, NK cells can destroy tumor cells without prior activation or priming. It is how NK cells were first discovered. They received their moniker from this 'natural' murder. NK cells release cytokines like IFN and TNF, stimulating the immune system by acting on immune cells like dendritic cells and macrophages.

NK cells are in continual touch with one another while on patrol. They activate and inhibit signals from receptors on the NK cell surface to control whether or not the NK cell destroys these cells. The chemicals expressed on the surface of cancer cells and infected cells are recognized by activating receptors, which then "turn on" the NK cell. Inhibitory receptors restrain NK cell killing. The majority of healthy, normal cells carry MHC I receptors, designating these cells as "self." The homologous MHC I is recognized by inhibitory receptors on the surface of NK cells, which switches off the NK cell and prevents it from killing.

As a result, NK cells can kill cancerous and diseased cells that have lost their MHC I. The target cell is lysed by the release of cytotoxic granules containing perforin and granzymes by the NK cell once the decision to kill has been taken.

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Role Of Natural Killer Cells

The main lymphoid tissue of the bone marrow is where natural killer cells are initially created. They go through positive and negative selection to weed out self-targeting cells. After adulthood, they relocate to secondary lymphoid tissues and undergo terminal maturation, per News Medical.

The activity of mature NK cells is regulated by stimulatory and inhibitory receptors, such as killer cell immunoglobulin-like receptors (KIR) and NKG2D. It is known that there are over 20 activation receptors that serve to recognize proteins that are not typically found on a cell's surface. Inhibitory receptors recognize MHC I proteins exclusive to the self. It is because numerous virus types have developed immunological evasion methods by downregulating the MHC class I, making it impossible for the immune system to recognize their viral antigens.

Overall, NK cells won't be activated, preventing the self-cell from being killed if there is a balance between inhibitory and stimulatory impulses. The inhibitory signals will outweigh the activation signals in this case. The NK cell will be activated if a high activation signal is caused by increased synthesis of stress ligands or a low inhibitory signal generated by decreased MHC I on the cell surface.