These are the final class of cells within innate immunity. They are not phagocytic, so they don’t engulf pathogens. The Natural killer Cells are probably the most versatile cell line within the innate immune system. These potent cells can kill tumour cells, cells that have been infected with viruses, bacteria, parasites, and fungi. The way in which they kill these, rather than engulfing and digesting them like phagocytic cells, is inducing them to commit suicide.
Natural Killer Cells can do this by two major means.
- The first method that they employ is almost like a type of lethal injection. They penetrate the surface of the target (tumour cell, infected cell etc) and using a substance called perforin, perforate the surface of the target. Once in, they deliver a burst of enzymes, such as granzyme B, that stimulate the target to commit suicide.
- On other occasions, a protein on the outer surface of Natural Killer Cells called Fas Ligand will bind to a protein called Fas on the outer surface of target cells. This binding will induce the production of self-destructing enzymes in the target cell.
So how do Natural Killer cells identify their targets?
Natural Killer Cells identify their targets with markers displayed on the target cell. It is a bit like a ‘kill’ or ‘don’t kill’ signal. Compounds such as MHC molecules on the outer surface of a cell gives a ‘don’t kill’ signal to the NK cells. Other compounds such as specialised carbohydrate or protein molecules interact with the surface of NK cells to instigate a ‘kill’ response.
NK cells work in tandem with macrophages.
During infection, for example, initial cytokines released will bind to NK cells and tell them that a battle needs to be waged. The NK cells will respond by producing other cytokines such as interferons, which activate macrophages. Activated macrophages as you remember, can recruit neutrophils. Activated macrophages also secrete substances such as TNF that can stimulate NK cells to increase their expression of interferons, and the whole response up-regulates itself and gets more aggressive.