Polysaccharide isotopes are responsible for many pathophysiological responses and can elicit strong immune responses. The first two Chapters demonstrated the chemical synthesis of a conserved and all α–linked Escherichia coli R3 outer core pentasaccharide. This pentasaccharide was conjugated to carrier protein (CRM₁₉₇) through a propyl amino linker at the reducing end. An immunological analysis demonstrated that this glycoconjugate can elicit specific anti-pentasaccharide antibodies with in vitro bactericidal activity. In Chapter three, a Core Synthesis/ Enzymatic Extension (CSEE) technique for building a comprehensive O-glycan library was proposed. Furthermore, a highly efficient and convergent methodology was designed to synthesize all eight O-glycan core structures in large scale. These two emerging techniques have great potential to enable the investigation of structure-activity relationship between glycans and receptor proteins, thus facilitating the identification of biomedically important glycan isotopes.