MHC class I molecules are one of two primary classes of major histocompatibility complex (MHC) molecules (the other being MHC class II) and are found on the cell surface of all nucleated cells in the body. Their function is to display fragments of non-self proteins from within the cell to cytotoxic T cells; this will trigger an immediate response from the immune system against the particular non-self antigen displayed with the help of 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 the cytosolic or endogenous pathway.