Vaccines based on synthetic peptides representing epitopes recognized by T-helper or cytolytic T-cells have been widely investigated as the basis for effective vaccines. However, their efficacy suffers from a number of important limitations. First, they are poor immunogens and exhibit short life-spans both in serum and within the cell. Secondly, the amount of peptide required to bind a threshold number of MHC class II molecules on the surface of an APC is reportedly higher than the minimal amount needed to bind to class II MHC molecules in the intracellular compartment in which the processing of the corresponding parent antigen takes place after internalization. Soluble antigens carrying CTL epitopes not only have the same drawbacks, but normally are unable to enter the appropriate intracellular compartment to undergo processing and presentation on class I MHC molecules. Thus, delivery systems based on immune-stimulating complexes, liposomes and synthetic lipopeptides have been designed in attempts to circumvent these limitations.