Shigella flexneri 3a causes bacillary dysentery. Its O-antigen has the {2)-[alpha-d-Glcp-(1 -> 3)]-alpha-L-Rhap-(1 -> 2)-alpha-L-Rhap-( 1 -> 3)-[Ac -> 2]-alpha-L-Rhap-(1 ->)-[Ac -> 6](approximate to 40%)-beta-D-GlcpNAc-(1 ->} ([(E)AB(Ac)C(Ac)D]) repeating unit, and the non-Oacetylated equivalent defines S. flexneri X. Propyl hepta-, octa-, and decasaccharides sharing the (E') A'BAcCD(E) A sequence, and their non-O-acetylated analogues were synthesized from a fully protected BAcCD(E) A allyl glycoside. The stepwise introduction of orthogonally protected mono-and disaccharide imidate donors was followed by a two-step deprotection process. Monoclonal antibody binding to twenty-six S. flexneri types 3a and X di-to decasaccharides was studied by an inhibition enzyme-linked immunosorbent assay (ELISA) and STD-NMR spectroscopy. Epitope mapping revealed that the 2(C)-acetate dominated the recognition by monoclonal IgG and IgM antibodies and that the BAcCD segment was essential for binding. The glucosyl side chain contributed to a lesser extent, albeit increasingly with the chain length. Moreover, tr-NOESY analysis also showed interaction but did not reveal any meaningful conformational change upon antibody binding.