Glycans are universal features of life, decorating virtually every cell surface and forming the dense, information-rich glycocalyx. Far from being passive structural elements, glycans orchestrate how cells sense, interpret, and respond to their microenvironment. They regulate immune recognition, microbial adhesion, leukocyte trafficking, and the discrimination between self and non-self antigens. Across biology, changes in glycan composition carry functional consequences: bacterial pathogens rely on surface glycans to define serotypes and evade immunity, while tumors remodel their glycocalyx to support growth, invasion, and immune escape. Despite their ubiquity and importance, tools capable of precisely probing or manipulating glycan-mediated immunity remain limited. In my dissertation, I leverage glyco-engineered outer membrane vesicles (OMVs) as modular platforms to dissect how defined glycan structures shape immune specificity across infectious and tumor contexts. I first examined Shigella dysenteriae OMVs to understand how native O-antigens guide humoral immunity. Immunization induced strong IgG responses with minimal off-target reactivity toward heterologous Shigella serotypes, along with protective efficacy in the bactericidal assay. I next explored tumor-associated glycosylation by applying a bacterial glycoengineering platform to install Tn glycans onto full-length mucin (MUC1) on living E. coli (to generate OMVs) and MUC1 OMVs. These glycoengineered OMVs retained innate and adaptive immune responses demonstrated by induced pro-inflammatory cytokine release and highly specific antibody responses to Tn-MUC1 with no off-target mucin reactivity. Although antitumor effects were modest due to low Tn expression in the tumor model, this approach establishes a precise and straightforward method for presenting cancer-associated glycoepitopes on OMVs. Together, these studies define how the structural logic of glycan presentation on OMVs governs immune specificity and functional outcomes. By integrating glycobiology with engineered bacterial vesicles, my work provides a design framework for generating precise anti-glycan immunity, advancing OMV-based vaccines, and illuminating the opportunities and limits of targeting glycans across infection and cancer.