Cytotoxic T cell
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A cytotoxic (or TC) T cell is a T cell (a type of white blood cell) which has on its surface antigen receptors that can bind to fragments of antigens displayed by the Class I MHC molecules of virus (or other intracellular pathogen) infected somatic cells and tumor cells.
Once activated by a MHC-antigen complex, TC cells release the cytoxins perforin and granulysin, which forms pores in the target cell's plasma membrane; this causes ions and water to flow into the target cell, making it expand and eventually lyse. TC also release granzyme, a serine protease, that can enter target cells via the perforin-formed pore and induce apoptosis (cell death).
A second way to induce apoptosis is through an interaction between cell-surface molecules on the TC and the infected cell. When a TC is activated it starts to express the surface cytokine FAS ligand, which can bind to Fas molecules on the target cell. This Fas-Fas ligand interaction is the main route to dispose of unwanted T lymphocytes during their development.
Most TC cells have the protein CD8 present on the cell surface, which is attracted to portions of the Class I MHC molecule. This affinity keeps the TC cell and the target cell bound closely together during antigen-specific activation. TC cells with CD8 surface protein are called CD8+ T cells.
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