Introduction
White blood cells are part of the body’s immune system, which helps in eradicating any infection or harmful foreign agents from the body. These are also known as leukocytes and are responsible for protecting the body from infections. These cells are also present in the blood and respond to any infection or injury as it happens. Natural killer cells are also a type of white blood cell that helps in the elimination of infected cells from the body. They are also a type of lymphocyte like B and T-cells. These cells are responsible for killing the harmful cells in the early stage, preventing cancer or infection from spreading.
What Are Natural Killer Cells?
Natural killer (NK) cells are the type of lymphoid cell that function in the immune system to remove infected or cancerous cells. These cells belong to a group of white blood cells called lymphocytes, which also include B and T-cells. These cells are called natural killer cells because they can destroy potential threats without getting exposed to them. However, on the other hand, cells like T-cells need previous exposure to any pathogen to destroy them or act against them.
Around five to ten percent of lymphocytes circulating in the blood are natural killer cells and have a short life span of around two weeks. At any given time, an adult may have two billion natural killer cells in their body.
What Are the Functions of Natural Killer Cells?
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Natural killer cells help in destroying the healthy cells that pose a threat to the body. These cells may be infected with viruses or may be cancer (malignant) causing cells. Therefore natural killer cells destroy these cells in the early stage and prevent them from spreading.
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Natural killer cells are the fighting cells and play a crucial role in the body's immune system. The immune system of the body is composed of physical barriers such as skin and other epithelial surfaces or mucous layers and the complement system and various cells such as phagocytes, mast cells, dendritic cells, and natural killer cells. These cells help in destroying any threat after they cross the barriers.
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The primary function of the natural killer cells is to destroy any harmful foreign substance in the body, but these cells can also communicate. These cells release a protein called cytokines that instruct other cells of the immune system to attack harmful cells and pathogens.
What Is the Role of Natural Killer Cells in the Innate Immune System?
The innate immune system defense against any foreign harmful agent or pathogen in the body. It responds to the threat much faster as compared to the acquired immune system and holds it off from causing any harm to the body. The natural killer cells are initially developed in the primary lymphoid tissue of the bone marrow (cell-forming tissue), where they function to remove self-targeting cells and get mature. After their complete maturation, these cells may patrol through the body in blood, scanning for any harmful bacteria or viruses and killing them by releasing deadly chemicals into the cells. The surface of natural killer cells possesses multiple receptors which control the activity of cells. These receptors work together to either stimulate (activate) or inhibit (prevent) the natural killer cell from killing a target cell. The whole process is explained below:
Inhibition -
Natural killer cells do not attack the healthy cells in the body as these healthy cells have a marker which is MHC-1 which makes the natural killer cell recognizes the cell as self and not attack them. The MHC-1 marker is attached to the natural killer cell inhibitory receptor, which inhibits the natural killer cell from killing the target cell, and the natural killer cell moves to the next cell. Some examples of inhibitory receptors include -
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NKG2A/CD94 provides inhibitory signaling for natural killer cells by binding with another HLA-E receptor expressed on the target cells.
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Killer-cell immunoglobulin-like receptors (KIRs) inhibitory pathway by binding with HLA- A, B, and C expressed on target cells.
The function of natural killer cells can be affected by various acquired and congenital immune deficiency disorders, and therefore these patients become highly susceptible to various infections such as human papillomavirus, herpes simplex, and cytomegalovirus.
Activation -
Natural killer cells activate to destroy the target cells which they do not recognize, which include -
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Certain cells that release activating signals, such as cancer cell some times release chemical signals that make the natural cells attack these cells.
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Natural killer cells can destroy the cells that do not express the MHC-1 marker, or in some cases, it may be downgraded, which means they are less amount of it present as compared to normal.
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Additionally, natural killer cells release certain proteins such as perforin and granzymes that helps in killing the target cells. Perforin helps in creating a window or opening in the target cell, and granzymes get into the cell and kill the cell.
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These activated natural cells may also release cytokines that help the other white blood cells to recognize the target cells and helps get rid of them from the body.
Some examples of activating receptors include -
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Natural cytotoxicity receptors (NCRs) NKp30, NKp44, and NKp46 (CD335).
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NKG2D/CD314 also triggers natural killer cell activation.
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NKG2C/CD94 gave activating signals to natural killer cells by engaging them with HLA-E receptors on the target cells.
What Is the Role of Natural Killer Cells in Cancer Treatment?
Cancer patients usually have low levels of natural killer cells, and researchers are working on it to provide enough natural killer cells to fight tumor cells by using the donor’s natural killer cells through allogeneic adoptive NK cell therapy. As these natural killer cells appear to be safe and do not cause any harmful reaction in the body. In addition to that, scientists are also looking at specific proteins that are either produced by these cells or are present in the body to regulate natural killer cells.
How to Improve the Functioning of Natural Killer Cells?
People can also improve the functioning of natural killer cells or improve their immune system by making some lifestyle changes such as -
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Exercising regularly can help improve the function of natural killer cells in people with cancer.
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Music therapy is also linked with the increased number and activity of natural killer cells.
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Cessation of smoking is one way to ensure that the natural killer cells are working normally, as smoking may interfere with their functioning.
Conclusion:
Natural killer cells are a crucial part of the human immune system as they help eliminate harmful viral-infected cells and cancer cells. These help in destroying the harmful cells in the initial stage, thus preventing their spreading in the body. A person should have a normal amount of natural killer cells in their body for the proper functioning of the immune system. Moreover, these cells provide an exciting possible treatment option for cancer as this targets just the cancerous cell and may kill these cells without causing any side effects.
