Bioactive components of human breast milk: Antimicrobial mechanisms, immunomodulatory properties, and laboratory relevance in neonatal care

Background : Human breast milk is a biologically complex fluid whose protective functions extend well beyond nutritional provision. The growing burden of antimicrobial resistance and neonatal vulnerability to infection have renewed interest in characterizing the antimicrobial and immunomodulatory components of human milk through laboratory methods. This narrative review synthesizes current evidence on the major bioactive constituents of human breast milk, with emphasis on mechanisms of action, laboratory quantification and translational relevance for neonatal care. Methods : A non-systematic literature search was conducted across international databases using terms encompassing lactoferrin, secretory immunoglobulin A, human milk oligosaccharides, antimicrobial peptides, exosomes, milk microbiome, resistome and donor milk processing. Priority was given to studies published in the past 10 years, with earlier foundational references included where appropriate. The substantial heterogeneity in sample collection protocols, measurement techniques and outcome definitions does not lend itself to meta-analytic synthesis; a narrative approach is therefore appropriate. Results : Human breast milk contains a multi-layered antimicrobial system of proteins (lactoferrin, lysozyme, antimicrobial peptides), secretory IgA, human milk oligosaccharides, extracellular vesicles, bacteriophages, and a resident microbiome. Conventional ELISA fails to resolve glycosylation-dependent functional parameters addressable only by mass spectrometry. HMO composition is 97% FUT2-genotype-determined, requiring structural profiling. Holder pasteurization reduces bioactive capacity and generates advanced glycation end products. Gestational age and metabolic status modulate concentrations clinically. Conclusions : Laboratory characterization of human milk bioactive components represents a clinically relevant and underexplored area of neonatal laboratory medicine. Standardized measurement protocols and evidence-based thresholds are needed to guide clinical decision-making and optimize donor milk banking.