Many glycan binding proteins (GBPs) are oligomeric and typically contain multiple sites for glycan binding (also called carbohydrate-recognition domains). The ability to form multivalent protein-ligand interactions significantly enhances the strength of binding: while  values for individual CRD-glycan interactions may be in the mM range, the overall affinity of GBP towards glycans may reach nanomolar or even picomolar ranges. The overall strength of interactions is described as avidity  (in contrast with an affinity  which describes single equilibrium). Sometimes the avidity is also called an apparent  to emphasize the non-equilibrium nature of the interaction.
Common oligomerization structures of lectins are shown below. For example, galectins are usually observed as dimers, while intelectins form trimers and pentraxins assemble into pentamers. Larger structures, like hexameric Reg proteins, may assemble into membrane penetrating pores. Collectins may form even more bizarre complexes: bouquets of trimers or even cruciform-like structures (e.g. in SP-D).