Cytotoxic T Cell

A cytotoxic T cell (also known as T_C, Cytotoxic T Lymphocyte, CTL, T-Killer cell, cytolytic T cell, CD8+ T-cells or killer T cell) is a T lymphocyte (a type of white blood cell) that kills cancer cells, cells that are infected (particularly with viruses), or cells that are damaged in other ways.

Most cytotoxic T cells express T-cell receptors (TCRs) that can recognize a specific antigen. An antigen is a molecule capable of stimulating an immune response, and is often produced by cancer cells or viruses. Antigens inside a cell are bound to Class I MHC molecules, and brought to the surface of the cell by the Class I MHC molecule, where they can be recognized by the T cell. If the TCR is specific for that antigen, it binds to the complex of the Class I MHC molecule and the antigen and the T cell destroys the cell.

In order for the TCR to bind to the Class I MHC molecule, it must be accompanied by a glycoprotein called CD8, which binds to the constant portion of the Class I MHC molecule. Therefore, these T cells are called CD8+ T cells.

The affinity between CD8 and the MHC molecule keeps the T_C cell and the target cell bound closely together during antigen-specific activation. CD8+ T cells are recognized as T_C cells once they become activated and are generally classified as having a pre-defined cytotoxic role within the immune system. However, CD8+ T cells also have the ability to make some cytokines.