Cells or cells that kill an injured or unhealthy cell are called Killer Cells. Cytotoxic killer cell (or Tc cell) and Natural killer cell (NK cell) both do this type of work. So these are two killer cells. We will now learn about natural killer cells.
Although Tc cell and NK cell are two killer cells, there is a difference between them. NK cells are always active and naturally destroy the injured cell. But Tc cells are not always active. They are activated through the helper cell, then they can destroy the injured cell.
NK cells are always in our blood and in different tissues. So if any other cell in the body is damaged, the NK cell can easily catch the signal that they create. A cell can produce two types of signals. + ve signal and -ve signal. If there is no pathogen in the cell or tissue, that cell or tissue + ve signals. But when a pathogen attacks there, it creates a cell or tissue -ve signal. If the signal is -ve, then the NK cell understands it and destroys the whole body of that cell or tissue so that the pathogens inside it can die.
When a cell is healthy, two things are attached to its surface. Class I MHC and AR ligand (a type of receptor). But when a pathogen or germ invades a cell, it has only AR ligand on the surface of the cell, not Class I MHC.
When NK cells move around the blood or different cells of the body, it keeps its two hands out. Both hands are called CD94 and AR (Androgen receptor).
When these two hands of the NK cell join the hands of the two healthy cells (e.g. Class I MHC and AR ligand), the NK cell takes this state as + ve signal. So he does no harm to those cells.
But when a cell is connected to an NK cell through a single arm (such as an AR ligand), the NK cell takes it as a -ve signal. So NK cell then releases some chemical to destroy that cell.
This means that the main feature of an unhealthy or injured cell is that there will be no MHC I type molecule on its surface.
The characteristics of viruses in microorganism are a little different. When a virus is inside a cell, it behaves like an organism, meaning that the virus is a living organism in the organism. So NK cell can easily destroy an unhealthy cell and also the virus inside it. But outside the cell, the virus behaves like an inert substance. So if the virus is outside the cell, then the NK cell cannot destroy it.
Inside an NK cell there are two main chemicals. These are-
i) Granzyme B and
When an NK cell attaches to an unhealthy cell, many Perforins from the NK cell form a pathway or channel in the cell membrane of that cell. In this way, Granzyme B from NK cells enters the unhealthy cell.
Granzyme B molecules work in unhealthy cells in such a way that the cell is destroyed automatically. This phenomenon is called cell autolysis or apoptosis.