A natural killer (NK) cell is part of the body’s immune system and is involved in destroying tumor cells or cells that are infected by viruses. 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.
A NK cell is a type of lymphocyte, or white blood cell, that is 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 that it does not recognize as healthy human cells; these might be pathogens, such as harmful bacteria, or they might be human cells that are not functioning normally because of infection or mutation. The innate immune system can be regarded as the body’s first line of defense, going into action while a more specific defense is organized, and NK cells play an essential part.
The NK cell has two types of receptors, structures that can bind to molecules present on the surface of a cell. There is an activating receptor that prompts the NK cell to kill a cell when it binds to any one of various molecules present on that cell’s surface and an inhibiting receptor that overrides the “kill” signal when it binds to a molecule called major histocompatibility complex I (MHC-I), which would be present on the surface of a normal cell. Cells that are cancerous or that are infected with a virus usually fail to produce MHC-I, so NK cells that encounter such a cell will not receive the inhibiting signal and will kill the cell.
Natural killer cells kill an infected or cancerous cell by releasing two types of chemicals. Perforins create small pores on the cell’s surface, then enzymes known as granzymes pass through the pores into the cell, stimulating the release of further enzymes which 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. The cell fragments are mopped up by other immune system cells known as phagocytes.
Two other types of chemicals also are released by NK cells. These are called cytokines and chemokines. They are involved in the activation of the adaptive immune response to infection and other threats.
Very rarely, NK cell leukemia resulting from an uncontrolled proliferation on natural killer cells can occur. It is thought to be associated with infection by Epstein-Barr virus. The role of NK cells in autoimmune disorders has been the subject of research in the early 21st century.