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 all 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.
