Distinct O-Linked Glycosylation Systems in Signaling and Immune Regulation.

O-linked glycosylation comprises distinct regulatory systems, including secretory-pathway mucin-type O-GalNAc glycosylation and intracellular O-GlcNAcylation. These modifications both target serine/threonine residues but differ in glycan structure, cellular compartment, enzymatic machinery, and biological function. This narrative review was based on targeted searches of PubMed, Web of Science, and related literature using keywords related to O-glycosylation, O-GalNAc glycosylation, O-GlcNAcylation, immune regulation, cell signaling, glycoproteomics, and congenital disorders of glycosylation (CDG). We summarize evidence that mucin-type O-glycosylation regulates receptor behavior, cell adhesion, immune checkpoints, immunoglobulin function, antigen recognition, and pathogen-host interactions, whereas O-GlcNAcylation mainly modulates intracellular signaling, transcriptional control, stress responses, post-translational modification crosstalk, and innate immune pathways. We also discuss how glycosylation defects, including CDG and selected O-linked glycosylation disorders, connect genetic variation with disease phenotypes. Recent advances in site-specific glycoproteomics, O-glycoprotease-assisted workflows, LC-MS/MS-based glycopeptide analysis, and spatial or temporal profiling have improved mechanistic interpretation but still face limitations in site localization, structural resolution, and functional validation. Overall, the evidence supports the hypothesis that distinct O-linked glycosylation systems act through different molecular mechanisms but converge on signaling regulation, immune homeostasis, and disease susceptibility.