The winged bean acidic lectin (WBA II) binds the H-type 2 human blood group related trisaccharide (α-L-Fuc-(1c → 2b)-β-D-Gal-(1b → 4a)-β-D-GlcNAc-OMe, 1). Interactions that importantly contribute to the specificity of the complex formation are provided by CH2-6b, OH-4b, OH-3b, and OH-2c of 1. On the basis of the relative activities of the monodeoxy and mono-O-methyl derivatives of 1, the hydroxyl groups at the 3a, 6a, and 4c positions become located at the periphery of the combining site, whereas the CH3O-1a, NHAc-2a, and CH3-5c groups are more remote from the protein surface and fully exposed to bulk water. Whereas the WBA II lectin marginally recognizes the H-type 1 related trisaccharide (α-L-Fuc-(1c → 2b)-β-D-Gal-(1b → 3a)-β-D-GlcNAc-OMe, 23), the de-N-acetyl derivative (24) is bound 3.4 times more strongly than 1. This and other results, which are related to changes in interaction between the ligand and the apron of the combining site, are attributed to changes in hydration that lead to enthalpy–entro...