A natural killer or NK cell is a white blood cell which acts as the immune system's first line of defense against foreign invaders like tumors, bacteria, and viruses. NK cells are known as “natural killers” because they don't need to react to specific antigens, they just need to recognize that a cell is foreign, and it does not belong. Because these cells are so powerful, the body has a number of steps in place which are designed to prevent NK cells from running rampant and accidentally attacking their host.
In order to understand how natural killer cells work, it is necessary to take a brief foray into biology. These cells look for proteins known as Human Leukocyte Antigens (HLAs) which are displayed on the surface of other cells. If a natural killer cell recognizes the HLA proteins as belonging to the “self,” it ignores the cell, assuming that it belongs. If, on the other hand, the HLAs appear foreign, the natural killer cell will release toxins which kill the foreign cell.
HLAs are expressed through the Major Histocompatibility Complex, a group of genes located on the sixth chromosome. The Major Histocompatibility Complex is “major” because it plays a critical rule in the acceptance or rejection of transplanted tissues. If the HLAs on an organ do not match, the body will reject it, with the natural killer cells moving in to protect the body from a perceived invader.
A natural killer cell has a number of receptors which it uses to gather information about the cells it comes into contact with. Typically, at least one specific receptor must be activated for a natural killer cell to take action. Other immune system cells may also get involved in the response, with the immune system swinging into action to address emerging health problems. The natural killer cell works by latching on to the suspect cell and essentially injecting it with cytotoxins, substances which are designed to kill cells.
Sometimes, things go wrong with natural killer cells. For example, sometimes a woman carries a baby with incompatible HLAs, and the natural killer cells think that the fetus is dangerous. In these instances, various medical treatments are used with the goal of allowing the baby to develop so that the woman can carry the child to term. Natural killer cells can also turn malignant, developing lymphoma, a type of cancer. NK cell lymphoma can be extremely difficult to treat.