Found throughout biology, polyvalent interactions are characterized by the simultaneous binding of multiple ligands on one biological entity to multiple receptors on another (top part of the illustration) and have a number of characteristics that monovalent interactions do not (bottom). In particular, polyvalent interactions can be collectively much stronger than corresponding monovalent interactions, and they can provide the basis for mechanisms of both agonizing and antagonizing biological interactions that are fundamentally different from those available in monovalent systems.