Understanding precisely how the gut microbiome shapes immunity—not just that it does—is now one of the most consequential frontiers in preventive medicine. For adults managing chronic inflammation, autoimmune risk, or metabolic dysfunction, this mechanistic clarity could eventually transform how microbiome-targeted therapies are designed and deployed.
This comprehensive review synthesizes current molecular evidence across six signaling axes connecting gut microbiota to the immune system. Short-chain fatty acids (SCFAs), bile-acid derivatives, and tryptophan catabolites each engage distinct host receptors—G-protein-coupled receptors (GPCRs), the farnesoid X receptor/TGR5 axis, and the aryl hydrocarbon receptor (AhR), respectively—to regulate epithelial barrier integrity and calibrate regulatory T-cell (Treg) versus Th17 polarization. Particularly notable is the review's treatment of riboflavin-derived metabolites presented via MR1 molecules, which shape mucosal-associated invariant T-cell (MAIT) development—a less-publicized pathway with significant implications for mucosal defense. IgA production at mucosal surfaces and systemic immune tone are shown to be downstream outputs of these coordinated signaling networks. Dysbiosis triggered by antibiotics, aging, or dietary disruption is framed not as a microbiome problem alone, but as a collapse of these receptor-mediated communication systems, linking it mechanistically to inflammatory, autoimmune, metabolic, and neuroimmune pathology.
What elevates this review beyond prior gut-immune syntheses is its simultaneous treatment of both the molecular granularity and the translational arc toward interventions—probiotics, prebiotics, synbiotics, fecal microbiota transplantation, postbiotics, and IgY-based passive immunotherapy. The evolutionary conservation angle across mammalian systems also adds useful context, suggesting these pathways are deeply robust rather than species-specific curiosities. The key limitation is inherent to reviews of this scope: mechanistic detail often derives from rodent models or in vitro systems, and causal directionality in humans remains incompletely established. Still, as a framework document, this is an unusually well-integrated map of a field that has historically suffered from fragmentation. For clinicians and researchers, it represents a valuable reference architecture.