Viruses are inter-cellular pathogen that infects the cells of the body. Degradation of virus protein inside the cell allows viral peptide to display on the surface of the cell bound to MHC class 1 protein.CD8 T cells, which recognize MHC peptide complexes, are activated and kill the infected cell. Having killed 1 cell the T cell can move to new target kill that cell and move on again
The killing process is initiated when the T cell receptors and CD8 together bind to MHC class 1 molecule bearing a viral peptide producing the signals that activate the T cell.
Cytotoxic T cell contain membrane vesicle called cytotoxic granules, which package the protein that kills target cells, The most important of these is protein called perforin and set of proteases called granzymes, these proteins are complexed with scaffolding protein called serglycin.
Activation of T cells causes the release content of the vesicles, delivering protein on the surface of the target cells,
Although the exact mechanism is not known the Perforin facilitates the delivery of the granzymes into the cytosol, at this point the target cell are destined for death and the T cell can migrate onward to find new target cells.
The process of cell death initiated by grandzymes uses the same basic cellular machinery that gives rise to apotosis. The granzymes target various cellular proteins that regulate apoptosis such as BID and pro-caspase-3, The granzymes cleaves the BID which in its truncated form causes cytocrome C to release from the mitochondria into the cytosol, simultaneously granzymes activates pro caspase -3 which in turn cleaves ICAD the inhibitor of CAD (caspase activated DNA)
The activated DNA migrates into the nucleus where it degrades the DNA ensuring the death of the cell.