Natural killer cell (NK cell)

Definition and function:

Belonging to leucocytes Natural killer cells (NK cells) are part of the innate immune system in humans. The natural killer cells are formed from lymphoid precursor cells in the bone marrow. Their task is to initiate the Programmed cell death (Apoptosis) in degenerated tumor cells, as well as cells that have been attacked by viral agents.
In contrast to T lymphocytes, NK cells do not recognize the foreign cells via antigens. In the cell membrane of the natural killer cells sit special, genetically determined, receptors for the recognition of peptide chains, so that signal proteins from own body cells do not lead to the activity of the killer cell. These genetically-determined proteins are known by the term MHC molecules or the major histocompatibility complex.
In the cell membrane are activating and inhibitory receptors. Only one cell with foreign signaling proteins activates the NK cell. Interleukins and interferons can additionally influence the activity.
Natural killer cells are indispensable for the immune system. Because they do not bind via antigen receptors, the NK cells recognize mutant cells that have not been recognized by the T lymphocytes, e.g. because the antigens on the cell membrane remain unchanged. However, both the tumor cell and a virus-affected cell inevitably produce extraneous signal proteins. In this context, the Missing-self hypothesis cited. This states that the NK cell induces apoptosis in a body's own cell only when something typical (genetically-determined) is missing in the peptide chain on the cell surface, not because something has been added or altered by the cell mutation. This is opposite to the recognition of antibody and antigen, the exact reverse mechanism for the recognition of foreign cells. In this way, the immune system is doubly secured.
Because of their ability to kill tumor cells in a targeted manner, research into natural killer cells may be leading the way in combating cancer.