MHC protein complexed with flu virus. Computer model showing the crystal structure of the human class II MHC (major histocompatibility complex) protein HLA-DR1 complexed with an influenza (flu) virus peptide. MHC class II molecules are cell surface molecules that are part of the immune system. They are found only on antigen-presenting cells and lymphocytes, where they present antigens to evoke the production of antibodies.