The invention relates to compounds having formula (I): Scaffold-[L-(Antigen)t]y (I) wherein Antigen represents at least a portion of a target antigen for modulating an immune response; wherein t is 0 or an integer of at least 1; wherein y is at least 1; wherein the number of Antigens on the Scaffold is at least 2; wherein L is a linking group or a covalent bond, wherein when L is a covalent bond, the covalent bond is a single bond attached to a sp or sp2 hybridized atom of the Scaffold and when L is a linking group, the linking group is attached to the Scaffold through a single bond attached to a sp or sp2 hybridized atom; whereby the Scaffold is sufficiently rigid to maintain the relative position of the single bonds attached to sp or sp2 hybridized atoms. Further described are compositions containing the compounds and methods of using them.