Cytotoxic T-cells is a sub group of T lymphocytes cells that are capable of inducting the death of infected somatic or tumor cells; they kill virus-affected cells, pathogens, and damaged or dysfunctional cells. Most cytotoxic T cells express T-cell receptors (TcRs) that can recognize a specific antigenic peptide bound to Class I MHC molecules, present on all nucleated cells, and a glycoprotein called CD8, which is attracted to non-variable portions of the Class I MHC molecule. The affinity between CD8 and the MHC molecule keeps the TC cell and the target cell bound closely together during antigen-specific activation. CD8+ T cells are recognized as TC cells once they become activated and are generally classified as having a pre-defined cytotoxic role within the immune system. Cytotoxic T cell activation When a virus attacks the cell, cell starts produces a viral proteins. The ubiquitin molecule tags these proteins and carries it to proteosome, where it digested into small peptide fragments. A peptidase enzyme breaks these fragments, the fragments further move to endoplasm reticulm and gain entry via TAP molecule. Inside the endoplasm reticulum it binds to developing MHC class 1 molecule (only if it has right conformation with it), MHC class1 molecule is carried to cell surface and embedded. This alerts CTL (cytotoxic tcells), which identifies the foreign virus protein in the MHC class 1 molecule.The T-cell receptor in CTL engages a conformational recognition along with the CD8 molecule, which leads to release Granzymes and perfornin to kill virus-affected cell.