Lipopolysaccharides (LPS), major virulence determinants in Gram–negative bacteria, are responsible for many pathophysiological responses and can elicit strong immune responses. In order to better understand the role of LPS in host–pathogen interactions and elucidate the immunogenic properties of LPS outer core oligosaccharide, an all α–linked Escherichia coli R3 outer core pentasaccharide was first synthesized with a propyl amino linker at the reducing end. This oligosaccharide was also covalently conjugated to a carrier protein (CRM197) via the reducing end propyl amino linker. An immunological analysis demonstrated that this glycoconjugate can elicit specific anti-pentasaccharide antibodies with in vitro bactericidal activity. These findings will contribute to further exploring this pentasaccharide antigen as a vaccine candidate. Human milk oligosaccharides (HMOs) are a family of diverse unconjugated glycans that exist in human milk as one of the major components. Characterization, quantification and biofunctional studies of HMOs remain a big challenge due to their diversity and complexity. The accessibility of homogenous HMOs library is essential to solve these issues which have beset academia for several decades. In this study, an efficient chemoenzymatic strategy, namely Core Synthesis/Enzymatic Extension (CSEE), for rapid production of diverse HMOs was reported. Based on 3 versatile building blocks and 4 robust glycosyltransferases, a library of 31 HMOs were chemoenzymatically synthesized and characterized by MS and NMR. CSEE indeed provides a practical approach to harvest structurally defined HMOs for various applications. Glycoproteins are extremely important for all life on the planet. Glycoproteins play important roles in various biological processes. Increasing evidences demonstrate that glycosylation of proteins could improve stability of proteins by stabilizing their tertiary structure and protecting them from proteolysis. Besides, glycosylation of proteins could provide targeting effects through glycan-lectin interaction. Furthermore, carbohydrates play crucial roles in humoral immunity in that many sugar epitopes are identified as antigens for antibodies. Glycoprotein could boost strong T cells mediated intercellular immune responses because homogeneous antigens present on the surface of proteins by multivalent bonds. In this study, the three advantages of glycoproteins, namely stabilizing proteins, targeting effects and eliciting immunological response, were extensively explored by broad collaboration with other groups.