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Cytotoxic T lymphocytes (CTLs), also known as killer T cells, provide a cell-mediated response to specific foreign antigens associated with cells. CTLs only respond to foreign antigen when it is presented bound to the MHC-1 expressed on the surface of cells. CTLs do not respond to soluble antigen, but induce apoptosis in viral-infected cells and in cancer cells. When the complex of antigen bound to MHC-1 is bound to antigen-specific T cell receptor, the cytotoxic T cell induces apoptosis in the target cell primarily by two pathways, one involving perforin-mediated apoptosis and the other involving Fas/Fas-ligand interaction. When CTLs are activated by recognition of specific antigen on a cell, they release perforin proteins that integrate into the membrane of the target cell and organize to form a membrane pore. This allows the protease granzyme to enter the cell and activate the apoptotic caspase proteolytic cascade, and also allows other molecules to cross the cell membrane and trigger osmotic lysis of the membrane. The interaction of T-cell Fas ligand with the Fas receptor in the target cell can also activate the caspase cascade and other pathways involved in apoptosis. The interaction of a CTL with antigen-MHC I complex activates the CTL to proliferate and amplify the clone of T cells that respond to that antigen, amplifying the immune response against that specific antigen. (This definition may be outdated - see the DesignNote.)
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