Once inside the target cell, they initiate a process known as programmed cell death or apoptosis, causing the target cell to die. Granzymes enter target cells through the holes made by perforin.
Enzymes called granzymes are also stored in, and released from, the granules. One of these mediators is perforin, a protein that can make pores in cell membranes these pores allow entry of other factors into a target cell to facilitate destruction of the cell. Cytotoxic factors are stored inside compartments called granules, in both cytotoxic T cells and NK cells, until contact with an infected cell triggers their release. When the NK cell finds a cell displaying fewer than normal MHC molecules it releases toxic substances, in a similar way to cytotoxic T cells, which kill the virally-infected cell.Ĭytotoxic cells are armed with preformed mediators. However, another immune cell specialises in killing cells that have a reduced number of MHC class I molecules on their surface – this cell is a natural killer cell or NK cell for short. If this happens, the T cell doesn’t know there’s a virus inside the infected cell. Some viruses stop MHC molecules from getting to the cell surface to display viral peptides. Viruses are highly adaptable, and have developed ways to avoid detection by T cells. The T cell releases cytotoxic factors to kill the infected cell and, therefore, prevent survival of the invading virus ( Figure 1). If the T cell receptor detects a peptide from a virus, it warns its T cell of an infection. Each cytotoxic T cell has a TCR that can specifically recognise a particular antigenic peptide bound to an MHC molecule. These proteins are called T cell receptors ( TCRs). Cytotoxic T cells have specialised proteins on their surface that help them to recognise virally-infected cells. One type of T cell is called a cytotoxic T cell because it kills cells that are infected with viruses with toxic mediators. If the cell is infected with a virus, these pieces of peptide will include fragments of proteins made by the virus.Ī special cell of the immune system called a T cell circulates looking for infections. To overcome this, cells employ a system that allows them to show other cells what is inside them – they use molecules called class I major histocompatibility complex proteins (or MHC class I, for short) to display pieces of protein from inside the cell upon the cell surface. Once inside, the cells of the immune system cannot ‘see’ the virus and therefore do not know that the host cell is infected. When a virus infects a person (host), it invades the cells of its host in order to survive and replicate.
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