The present invention provides for novel immungens that are comprised of an activated polyhydroxypolymer backbone to which is attached 2 separate antigenic determinants. The 1st antigenic determinant includes a B-cell or CTL epitope and the 2nd antigenic determinant includes a T-helper epitope. In preferred embodiments, the antigenic determinants are derived from different molecules and species. Exemplary immunogens of the invention are constituted of a linear tresyl-activated dextran backbone to which is coupled B-cell or CTL epitopes of an antigen and to which is also coupled universal T-helper epitopes. Also disclosed are immunogenic compositions comprising the immunogens, methods of immunization and a method for identification of suitable immunogens of the invention.