Natural killer cell

The natural killer cell is a type of lymphocyte, a defense cell found in the blood. It is called a large granular lymphocyte because it is fairly large and carries granules, small packets visible inside the cytoplasm. It works in innate immunity, the defense that acts from the very start, against viruses, parasites and cells that have turned into tumor cells.

The interesting part is how it decides. On its surface it carries two kinds of receptors, activating ones and inhibitory ones, that read the surface of the other cells in the body. The inhibitory ones recognize the signals that say a cell is normal and belongs to the body; as long as they see them, they win out and hold the attack back. It checks that every cell is still showing its self ID badge.

That balance breaks when a viral infection or tumor transformation changes the molecules on that surface. The activating receptors then take over, and the natural killer cell releases granzyme and perforin, and with them induces apoptosis in the target cell: programmed cell death, an orderly shutdown rather than an explosion.