the invention concerns the use , in the induction of an immune response , of a synthetic microparticle polymer carrying on the surface one or more covalently bonded proteins capable of carrying one or more epitopes , the densities of the protein on the surface of the microparticles , and their molecular weights , being adjusted so as to direct the immune response to the induction of a humoral and cellular response or to the induction of a largely cellular response . said microparticles have an average diameter of approximately 0 . 25 to 1 . 5 μm .