Cytotoxic T cell

A cytotoxic T cell is a lymphocyte, a kind of white blood cell, that destroys the body's own cells once they become infected or abnormal. It carries a surface protein called CD8 and a T cell receptor that recognizes one particular antigen, a small piece of foreign protein. Like other T cells, it arises in the bone marrow and matures in the thymus, where cells that react strongly against the body's own proteins are weeded out.

Nearly every nucleated cell displays fragments of its internal proteins on MHC class I molecules. When a virus or a cancer-causing change produces unusual proteins, their fragments appear on the surface, and a matching cytotoxic T cell binds to them. Once activated, usually with support from helper T cells, it divides into many identical copies. Each copy releases perforin, which opens pores in the target's membrane, and granzymes, enzymes that pass through those pores and trigger apoptosis. This orderly cell death keeps the pathogen from spilling out to infect neighbors.

Some of the new cells persist as memory cells that respond faster on reinfection. Students meet cytotoxic T cells in immunology courses and in cancer immunotherapy, where they can be grown in the lab and given to patients to attack tumors.