Natural killer cells are part of the immune system and can recognize and destroy cancerous or infected cells. They are regulated by the immune system and release chemicals to kill cells by programmed cell death. NK cells are part of the innate immune system and have two types of receptors. They release cytokines and chemokines to activate the adaptive immune response. NK cell leukemia can occur, and their role in autoimmune diseases is being researched.
A natural killer (NK) cell is part of the body’s immune system and is involved in the destruction of cancer cells or virus-infected cells. NK cells can kill the body’s own cells, so NK cell activity is carefully regulated by the immune system. Natural killer cells can recognize and target infected and cancerous cells but do not normally attack healthy cells.
An NK cell is a type of lymphocyte, or white blood cell, produced by the bone marrow and is part of the innate immune system rather than the adaptive immune system. The adaptive immune system generates a specialized response to a particular type or strain of infecting organism, and the innate immune system responds to non-specific threats by attacking cells it does not recognize as healthy human cells; these could be pathogens, such as harmful bacteria, or they could be human cells that are not functioning normally due to infection or mutation. The innate immune system can be considered the body’s first line of defense, kicking into action as a more specific defense is organized and NK cells play an essential role.
The NK cell has two types of receptors, structures that can bind to molecules on a cell’s surface. There is an activating receptor that prompts the NK cell to kill a cell when it binds to any of various molecules on that cell’s surface and an inhibitory receptor that overrides the “kill” signal when it binds to a molecule called a complex major histocompatibility I (MHC-I), which would be present on the surface of a normal cell. Cells that are cancerous or infected with a virus usually fail to produce MHC-I, so NK cells that encounter such a cell will not receive the inhibition signal and will kill the cell.
Natural killer cells kill an infected or cancerous cell by releasing two types of chemicals. Perforins create tiny pores on the surface of the cell, then enzymes known as granzymes pass through the pores into the cell, stimulating the release of more enzymes that kill the cell by apoptosis, which is programmed cell death. During apoptosis, the cell shrinks and fragments, with each cell fragment contained within a membrane. This prevents the release of viruses or harmful substances when the cell dies. Cell fragments are taken up by other cells of the immune system known as phagocytes.
Two other types of chemicals are also released by NK cells. These are called cytokines and chemokines. They are involved in activating the adaptive immune response to infections and other threats.
Very rarely, NK cell leukemia resulting from uncontrolled proliferation on natural killer cells can occur. It is thought to be associated with Epstein-Barr virus infection. The role of NK cells in autoimmune diseases has been the subject of research in the early 21st century.
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