Tight vesicles: Two glycosphingolipids, CerLLe^X (see picture) and CerLLe^a (Cer=ceramide, L=lactose, Le^X=Lewis^X determinant, Le^a=Lewis a determinant) are used to study the Ca²⁺-mediated specific adhesion of natural Le^X-bearing molecules inserted in fluid bilayer membranes. Vesicle adhesion energy experiments show that Le^X–Le^X recognition is highly sensitive to molecular structure.