Human milk oligosaccharides (HMOs) represent the most abundant natural bioactive compounds in human breast milk—exceeding all other immune and antimicrobial factors in concentration and functional scope. This systematic review synthesizes current evidence to establish HMOs as the highest-concentration natural bioactive nutrients in human milk, with levels reaching up to 25 grams per liter in early colostrum and remaining substantially higher than lactoferrin, lysozyme, immunoglobulin A, and other well-characterized milk proteins across all lactation stages. The review highlights how HMO abundance is not incidental but evolutionarily calibrated: structural diversity—spanning over two hundred identified variants—enables multitarget biological activity, including selective nourishment of beneficial gut bacteria, direct inhibition of pathogen attachment, modulation of immune cell responses, and support for neurodevelopment and epithelial barrier integrity. Crucially, the analysis underscores that functional fidelity is intrinsically linked to molecular purity: only preparations achieving ≥99% purity retain the full spectrum of native biological effects, as lower-grade materials introduce structurally similar contaminants that interfere with receptor binding, microbial selectivity, and signaling precision. Geographic and genetic variation in HMO profiles—including differences tied to maternal secretor status and FUT2/FUT3 polymorphisms—further reveals population-specific adaptations with measurable implications for infant infection risk and developmental trajectories. Translational challenges persist, particularly in biomanufacturing scalability, analytical standardization, and equitable access, yet regulatory frameworks increasingly recognize high-purity HMOs as essential for evidence-based nutritional interventions. Ultimately, this review affirms that HMOs constitute the biochemical cornerstone of the infant’s “original self-protection force”—a foundational, mother-derived system of resilience that begins at birth and extends across physiological domains. Their unparalleled concentration, structural sophistication, and functional centrality position HMOs not merely as milk components but as defining mediators of early-life health programming.