The K cell, also known as the killer cell or natural killer cell, is considered a fundamental part of the body's innate immune system. These cells help to protect the body from illness by finding and killing abnormal cells that occur in tumors, infected cells, and invasive, parasitic, or foreign cells. The K cell not only kills infected or abnormal cells within the body, but can also release the enzymes that call other types of immune cells, such as T and B cells, to a specific area of the body to combat illness or infection. The K cell is considered especially important in helping the body to fight off viral infection.
K cells are part of innate immunity, meaning that they exist as an integral part of a healthy immune system. The K cell typically accounts for 10 to 15% of a healthy person's white blood cells, or lymphocytes. These cells are often the first to recognize pathogenic invaders within the body, and, as such, they can be crucial for initiating the immune response.
The K cell is considered cytotoxic. When it encounters a tumor cell, bacteria, other foreign cell, or a cell infected by a virus, it kills that cell by releasing a protein called perforin. Perforin punches holes in the offending cell's outer membrane. The killer cell can then release granzyme, a protease that penetrates the punctured cell membrane and causes apoptosis, or cell death. Not only is the infected or invading cell usually killed, but any viruses replicating inside of it are generally also killed.
Once the abnormal, infected, or foreign cell is destroyed, the killer cell can release enzymes that call T cells and B cells, other important white blood cells, to the area to help kill the tumor or fight off the infection. These cells, particularly the T cells, can produce enzymes of their own that can make the action of K cells even more effective.
Natural killer cells are particularly important in fighting off infection by viruses, since their means of inducing cell death by apoptosis can help prevent the spread of the virus to other cells of the body. Other immune cells can kill cells infected by viruses, but they often do so by cell lysis, a process that releases the replicated viruses within the affected cell. Once released, the replicated viruses can spread to other, nearby cells and make the infection worse.