MHC class I molecules are one of two primary classes of major histocompatibility complex (MHC) molecules and are found on the cell surface of all nucleated cells in the bodies of vertebrates. Their function is to display peptide fragments of non-self proteins from within the cell to cytotoxic T cells; this will trigger an immediate response from the immune system against a particular non-self antigen displayed with the help of an MHC class I protein. Because MHC class I molecules present peptides derived from cytosolic proteins, the pathway of MHC class I presentation is often called cytosolic or endogenous pathway.