The present invention provides an MHC class II antigen presentation enhancing hybrid polypeptide. The hybrid has an N-terminus comprising the mammalian Ii key peptide LRMKLPKPPKPVSKMR (SEQ ID NO: 1) and modifications thereof which retain antigen presentation enhancing activity, a C-terminus comprising an antigenic epitope in the form of a polypeptide or peptidomimetic structure which binds to the antigenic peptide binding site of an MHC class II molecule, and an intervening chemical structure covalently linking the N-terminal and C-terminal components.