MHC protein-antigen complex. Computer model of the human class I MHC (major histocompatibility complex) protein HLA-D27 complexed with a peptide antigen. MHC class I molecules are cell surface molecules that are part of the immune system. They are found on every nucleated cell of the body. They present protein fragments (peptides) from within the cell to T-cell immune cells. If foreign proteins are found, for example from a virus that has infected the cell, the T-cells become activated and kill the cell.