The Mother—Infant Symbiosis: A Novel Perspective on the Newborn’s Role in Protecting Maternal Breast Health

Abstract

Breastfeeding is a complex biological system and a bidirectional physiological dialogue in which the infant may contribute to maternal breast health. This review synthesizes current evidence, clearly separating established findings from emerging hypotheses, to examine the possible infant-driven mechanisms that influence hormonal and immune homeostasis in the mammary gland. We evaluate how neonatal suckling coordinates interconnected hormonal reflexes and immune activity, and we explore the hypothesis that the retrograde flow of infant saliva to the breast tissue could activate maternal enzymatic defenses, particularly the xanthine oxidase and lactoperoxidase systems. We also consider the activation of antimicrobial peptides through direct contact at the nipple and areola, including cathelicidin and defensins, as well as the potential roles of fetal microchimerism and microbial transfer from the infant’s mouth in strengthening breast resilience. Although much of the evidence remains indirect and based on in vitro and animal models, the convergence of data supports a reformulated conceptual model that presents the infant as an active physiological partner rather than a passive recipient of milk. Recognizing this shift has important clinical implications for the prevention of inflammatory conditions such as mastitis, the improvement of breastfeeding support strategies, and the optimization of maternal and infant health outcomes. The review also identifies significant gaps in current knowledge and cautiously proposes hypotheses to explore these mechanisms. While preliminary, this framework offers an original perspective that may guide future research and open new paths in the study of human lactation biology.

1. Introduction

Exclusive maternal breastfeeding is recognized worldwide as the gold standard for child nutrition, as it provides essential macronutrients and a complete range of biomolecules (cells and immune factors, growth modulators, RNA, miRNA, and microbiome components) [1,2,3,4,5,6]. Thanks to this rich content, maternal milk significantly reduces the risk of infectious diseases, including respiratory and gastrointestinal infections [7,8]. In addition to these immediate benefits, maternal breastfeeding has long-term effects that promote cognitive and emotional development from childhood [9,10]. It has long-lasting metabolic effects that translate into a lower prevalence of obesity and chronic illnesses such as type 2 diabetes and hypertension, as well as lower mortality in adulthood [11]. In addition to the benefits of maternal breastfeeding for the newly born, the mother also obtains multiple benefits, such as less postpartum bleeding, better glucose and insulin regulation, and a healthier lipid profile [12,13,14]. Over the long term, prolonged maternal breastfeeding is also associated with a lower risk of ovarian and even breast cancer [15,16].

These maternal advantages actually represent the return on a physiological investment made by the mother’s body during pregnancy and breastfeeding. A central component of this adaptation is the transformation of the mammary gland. Under the influence of estrogen, progesterone, and prolactin, the breast tissue changes drastically from a latent state to a highly secretory one [17,18,19]. The ductal and lobuloalveolar systems expand, and vascularization increases to favor milk synthesis and nutrient transport to the epithelium [20].

This complex process culminates in lactogenesis, which unfolds in two stages: lactogenesis I, which begins around the 16th week of pregnancy with the production of colostrum, and lactogenesis II, which starts after childbirth with a sudden drop in progesterone levels, which causes an abundant secretion of milk [21,22].

However, milk production is only part of the equation; its ejection and maintenance depend critically on the participation of the newborn. Breastfeeding suction is not only a mechanical action; it also serves as a potent stimulus for prolactin (PRL) and oxytocin (OXT) release, which are essential for sustaining milk production and ejection. Maternal breastfeeding induces a deep physiological calm in the mother through tactile stimulation of the infant, mediated by parasympathetic activation, and not directly by breast milk, which promotes an efficient and cooperative interaction [23,24].

However, this physiological balance can be compromised. Recent advances in lactation science also reveal that inadequate milk drainage or excessive intraductal pressure can alter the tight connections between lactocytes, triggering localized inflammatory responses that evoke a spectrum of clinical conditions grouped at the end of benign breast inflammation associated with lactation (LABI), including ductal obstruction, non-infectious mastitis, and breast abscesses. These conditions are better understood thanks to an emerging model that explains how mechanical forces and biological responses interact in the lactating breast [25,26,27]. Taken together, these findings provide important insights into the pathophysiology of these disorders but also paint a picture of a mammary gland in a profoundly dynamic, receptive, and vulnerable state during lactation [28]. After this, the gland enters into involution, characterized by epithelial regression, apoptosis, and stromal remodeling, a process like tissue repair that can have long-lasting implications for breast health and cancer risk modulation [29].

While previous research has clarified important aspects of maternal adaptations, it has also reinforced the conventional view of breastfeeding as a unidirectional process in which the infant is portrayed as a passive recipient. This perspective overlooks the complexity of the symbiotic relationship and fails to account for the significant contributions of the newborn to breast homeostasis. In response, this critical review proposes a paradigm shift in the conception of breastfeeding, advancing the idea that lactation should be understood as a co-regulated system rather than a one-way transfer of nutrients and immune protection to the newborn. At the core of this hypothesis is the recognition of the infant as an active and indispensable companion of the maternal breast’s physiological and immunological state. Beyond addressing this conceptual gap, the novelty of this work lies in its integrative synthesis: while previous reviews have often focused on isolated elements—such as the composition of the milk microbiome, the regulation of lactation by maternal hormones, or the mechanical aspects of suckling—this review unites these perspectives by highlighting the biomechanical, biochemical, and microbial mechanisms through which infants may exert an active influence on breast health. Taken together, this approach seeks not only to reframe the traditional understanding of breastfeeding but also to emphasize the bidirectional dialogue that underpins its biological and clinical relevance.

To build this new framework and critically evaluate the evidence that supports this hypothesis, we carried out a structured bibliographic review. An exhaustive search was carried out in the PubMed, Scopus, and Web of Science databases to identify relevant articles published until July 2025. The search strategy was designed to use a combination of keywords such as “immunity during breastfeeding”; “Neonatal immunomodulation”; “Breast inflammation”; “Xanthine oxidase”; “Antimicrobial peptide in human breast milk”; “Retrograde flow”; “Fetal microchimerism”; “Maternal milk microbiota”; “Maternal–infant symbiosis”, carefully distinguishing between theories and theories that are plausible but not proven. We prioritize human studies (in vivo and in vitro) but include fundamental animal studies where human data are lacking, explicitly highlighting the limitations of extrapolation. This systematic strategy ensured the inclusion of the most relevant and up-to-date evidence regarding maternal–infant interactions during breastfeeding.

The synthesis of the collected evidence enables the proposal of a novel conceptual framework that identifies the main gaps in current knowledge and provides a roadmap for future investigations. This framework emphasizes the bidirectional and mutualistic nature of breastfeeding, moving beyond the traditional unidirectional perspective. Such an approach has direct clinical relevance, as a deeper understanding of these dyadic interactions may inform innovative strategies for the prevention and management of breast inflammatory diseases. Moreover, recognizing maternal breastfeeding as a complex, co-regulated system opens new opportunities to improve health outcomes for both mother and child, laying the groundwork for more personalized and effective breastfeeding support strategies.

2. Hormonal Protection of the Mammary Gland Induced by Neonatal Stimuli During Lactation

Breastfeeding is a bidirectional process in which the physical drive of neonatal suckling orchestrates a maternal hormonal profile essential for breast protection and immunity, a response dictated by the newborn [30,31]. Beyond milk production, this interaction triggers key neuroendocrine-immune pathways involving OXT, PRL, and progesterone, whose modulation is also influenced by stress, reassurance, and multiparity. Among these molecules, OXT emerges as a central axis of this communication.

OXT, released centrally and peripherally by suckling, is a potent immunomodulator that functions as an endogenous “stress-coping” molecule with marked anti-inflammatory and antioxidant properties [31,32] (Figure 1).

Its receptors (OXTR), present on key immune cells such as macrophages and lymphocytes, are upregulated during immune challenges, allowing OXT to act directly to attenuate innate activation. At the molecular level, OXT inhibits the production of proinflammatory cytokines such as IL-1β and TNF-α and suppresses the NF-κB signaling pathway, thereby promoting homeostasis and immune surveillance in the tissue [33]. Recent evidence demonstrates that OXT predicts infant salivary secretory IgA (s-IgA) levels, acts locally in the gut via OXTR receptors to strengthen the epithelial barrier, and provides an initial antimicrobial shield, creating a defensive system initiated by the mother and calibrated to infant demand. Experimental statistical model demonstrates the context-dependent influence of OXT on infant social behavior indirectly mediated by salivary s-IgA: under conditions of maternal anxiety, increased milk OXT correlates with higher infant s-IgA levels but lower social interaction, indicating a stress-induced shift toward immune surveillance rather than behavioral exploration. This same OXT-sIgA axis simultaneously protects maternal mammary tissue by reducing upstream pathogen colonization, potentially decreasing the risk of mastitis [34].

A recent meta-analysis of 29 clinical studies confirms that neonatal sucking induces an immediate and pulsatile release of OXT in the mother, with up to five pulses in the first 10 min of breastfeeding during early postpartum. These pulses, which are short-lived (≈20 min), are associated with increased milk production and prolonged breastfeeding duration. In addition, the released OXT exerts anti-stress effects by reducing ACTH and cortisol and promotes maternal psychological adaptation by promoting sociability and reducing anxiety [35]. A study in rats demonstrated how cesarean section stress perturbed plasma OXT and immune balance, with effects reversed by subcutaneous administration of exogenous OXT (0.4 mg/kg) [36]. In humans, maternal psychosocial status, under hormonal regulation, influences mammary immunoprotection: factors such as anger or positive events correlate with higher secretory immunoglobulin A (SIgA) in human milk (hBM), whereas perceived stress decreases it. Secretory immunoglobulin A (SIgA) in human milk (hBM) and serum markers (IL-6, TNF-α, IFN-γ, cortisol) are significant predictors of SIgA, and crucially, psychosocial factors also modulate SIgA and were correlated with higher levels. At the same time, perceived stress was inversely associated [37]. Thus, the available clinical evidence complements the experimental findings, consolidating the role of OXT as a central mediator in neonatal interaction-induced mammary immunomodulation [31].

PRL, which is primarily triggered by tactile stimulation by neonatal suckling [23], also plays a dual role in lactogenesis and mammary immunomodulation (Figure 1). Prolactin (PRL) is an antagonist of the YAP-CCN2 oncogenic axis, activating the Hippo breast cancer suppressor pathway. Inhibition of YAP prevents its nuclear translocation, repressing the expression of key protumorigenic target genes (CCN2), the aggressive phenotype of cancer cells, and maintaining standard epithelial architecture through proper localization of apical-basal polarity protein complexes (Par, Crumb, and Scrib). The findings were made using multiple human breast cancer cell lines, including MCF7, and validated in several human datasets. A possible future line of research focusing on breast milk epithelial cells and the role of the newborn in maternal mammary health during lactation could involve a cohort study in lactating humans, which would allow for the investigation of the PRL/PRLR and Hippo/YAP pathways directly in a physiological context, by analyzing breast milk samples and mammary epithelial cells from mothers with and without cancer risk factors [38].

Investigations with rat peritoneal macrophages in vitro (neurochemically profiled by HPLC-ED and hormonally by radioimmunoassay) revealed that, despite lower basal serum PRL levels in multiparous rats, their macrophages exhibit increased and faster sensitivity to exogenous PRL, with enhanced oxidative burst and phagocytosis [39]. This adaptation, influenced by multiparity, is linked to reduced hypothalamic DOPAC levels and dopamine turnover in primiparous rats [40]. Pharmacological induction of hyperprolactinemia with domperidone in virgin rats confirmed that increased PRL directly enhances macrophage oxidative and phagocytic function [40]. Furthermore, PRL modulates mammary epithelial immunity: in bovine cells, it activates NF-κβ to enhance innate defense [41], and together with 17 β-estradiol (E2), it enhances innate immunity through epigenetic reprogramming (increased H3K9Ac and H3K9me2) [42]. PRL also promotes the migration of immune cells (monocytes/macrophages via CCL2 and neutrophils via CXCL1) into the mammary environment [40].

In short, intricate hormonal pathways, fundamentally dictated by the interaction with the newborn, orchestrate sophisticated cellular adaptation and specialization in the mammary gland, strengthening its resilience. A comprehensive scRNA-seq study in human milk (hBM) revealed cellular dynamics and differential hormonal regulation key to mammary adaptation. While secretory lactocytes (LC2) upregulated their estrogen (ESR1) and prolactin receptors (PRLR) to sustain milk production, LC1 downregulated ESR1/PRLR, reducing STAT5 signaling and lactate genes [43]. This molecular finding suggests that hormonal signals drive cellular specialization and functional adaptation of the mammary gland, thereby enhancing its resilience during lactation. It further emphasizes that lactation is a dynamic process involving coordinated responses from the nervous and immune systems, in which the newborn plays a pivotal role, with protective implications for mammary health in both the short and long term.

3. Functional Symbiosis of Montgomery Glands at the Beginning of Lactation

Areolar Tubercles or Montgomery glands are sebaceous-apocrine structures numbering 4–28 per areola that undergo marked hypertrophy during pregnancy and lactation, a clear adaptive response to enhance nipple–areolar protection and functionality [44,45]. While their secretory lipid film—comprising triglycerides, wax esters, cholesterol, squalene, and free fatty acids—acts as a biochemical barrier with proven antimicrobial effects against Staphylococcus aureus and gram-positive cocci [46,47,48], emerging evidence highlights their pivotal role as sensory and developmental interfaces shaped by the neonate. The bilateral hypertrophy of these glands, confirmed histologically as sebaceous ectopia, with oily secretion, demonstrates the plasticity of the tubercles during lactation and supports the notion that their lipid secretions are part of a local protection system against inflammation and microbial colonization [49].

An ex vivo study applying areolar secretions to the lips and nostrils of 3-day-old infants (n = 16) induced a robust increase in inspiratory amplitude, oral search behavior, and rooting—even without prior breastfeeding experience—demonstrating that volatiles (medium-chain fatty acids, aldehydes, ketones) serve as innate chemosensory cues facilitating early latch [50,51]. Furthermore, data from a cohort of 121 mother–infant dyads showed that a higher areolar tubercle count correlated positively with faster initiation of lactation and greater neonatal weight gain—infants of primiparous mothers with fewer glands had slower weight gain and delayed milk onset [52].

These findings propose the neonate as a recipient and an active participant in glandular development and maintenance. Suckling stimulates areolar mechanoreceptors, triggering hypothalamic OXT and PRL release [53]. However, the integrity of Montgomery glands appears to modulate localized anti-inflammatory responses further, helping to protect against subareolar inflammation during early lactation [54].

Improper care—such as friction, comedogenic agents, or aggressive suction—disrupts glandular function, increasing the risk of benign LABI and mastitis. Clinically, active maintenance of Montgomery glands (via gentle hygiene, avoiding ointments that occlude) is associated with an easier latch, fewer inflammatory episodes, and extended breastfeeding duration [51,54].

Overall, areolar Tubercles function as embryonic bio-immunological and sensorial hubs at the breast–infant interface. Infant interactions—mechanical and chemosensory—support gland development and functional integrity, optimizing microbial defense, lactation performance, and maternal comfort by reinforcing both physical lubrication and sensory-driven feeding initiation.

4. Xanthine Oxidase: A Mediator of Innate Immunity in the Mammary Epithelium

Xanthine oxidoreductase (XOR) is an iron-sulfur-molybdenum flavoenzyme, a homodimer (~300 kDa), key to purine metabolism. It is widely distributed in mammalian tissues, including lactating mammary epithelium. It is currently recognized as a significant component of the milk fat globule membrane [55,56,57,58].

In mammals, XOR is found in two interconvertible forms: xanthine dehydrogenase (XDH) and xanthine oxidase (XO). Only XDH can reduce NAD+, making it its preferred electron acceptor. Meanwhile, XO reduces molecular oxygen, generating superoxide anion (O₂⁻) and hydrogen peroxide (H₂O₂) [56]. It can also generate nitric oxide (NO) from nitrite under hypoxic conditions [59,60]. The generation of O₂⁻ and H₂O₂ activates various oxidative pathways that exert bactericidal and bacteriostatic effects [55]. The particular composition of newborn saliva likely influences the activation of this innate immune system. Experimental research has quantified that the concentrations of the XO substrates, hypoxanthine and xanthine, in neonatal saliva (medians of 27 and 19 µM, respectively) are approximately ten times higher than those found in adults (2.1 and 1.7 µM). This high availability of substrates is essential, since when mixed with breast milk, which contains high XO activity in the milk fat globule membrane, a robust production of H₂O₂ is triggered. Mixing neonatal saliva with breast milk increases the concentration of H₂O₂, and this mixture has demonstrated an inhibitory effect on the growth of S. aureus and E. coli [61]. Breast milk and neonatal saliva generate an additional amount of >40 µM H₂O₂ to that already present in milk (~27 µM). This concentration is biologically active and exerts a selective antimicrobial effect: the tests showed a dose-dependent inhibition of the growth of opportunistic pathogens such as Staphylococcus aureus and Salmonella spp. Continuous H₂O₂ generation has been thought to contribute to the innate immune protection of the mammary gland during lactation, particularly in preventing S. aureus mastitis and E. coli infections [62,63]. The antimicrobial activity in humans of this enzymatic system is dynamically regulated in a human mammary epithelial cell model, showing that while basal XOR activity is low, it is significantly upregulated by inflammatory cytokines, allowing for a swift amplification of their antimicrobial function without requiring new protein synthesis [64].

The H₂O₂ generated by XO activates lactoperoxidase (LPO), present in both milk and saliva, to produce hypothiocyanite (OSCN⁻), an even more potent antimicrobial agent [65]. Interestingly, neonatal saliva not only contributes the activators (hypoxanthine and xanthine), but also nucleosides and bases that selectively promote the growth of commensal bacteria such as Lactobacillus spp., while inhibiting pathogens. This dual mechanism—inhibiting pathogens and promoting commensal-illustrates a unique biochemical symbiosis, where infant saliva not only extracts nutrients, but also actively regulates innate immunity at the maternal–infant interface [65], to produce OSCN⁻ from the oxidation of pseudohalide thiocyanate (SCN⁻). OSCN is also a potent antimicrobial agent, so XO and LPO are thought to act together to generate reactive oxygen species (ROS) and reactive nitrogen species (RNS) [55]. The presence and functionality of LPO in breast milk have been confirmed using highly sensitive immunochemical and biochemical methods to quantify LPO activity in different milk species, eliminating interference from endogenous H₂O₂ and SCN⁻ and normalizing samples through the exogenous addition of SCN⁻. This approach allowed for accurate comparative evaluation, confirming that human milk contains functional levels of LPO, capable of participating in the formation of OSCN⁻ when activated by H₂O₂ generated by XO. These findings reinforce the relevance of the XO-LPO system as an active antimicrobial mechanism in the human mammary gland during lactation [49,66] (Figure 2).

These data suggest a complementary innate immune defense mechanism in the mammary epithelium induced by the retrograde flow of hypoxanthine- and xanthine-rich neonatal saliva due to the retrograde flow of infant saliva into the mammary ducts during suckling [67]. This mechanism would involve the activation of xanthine oxidase and the subsequent production of ROS and RNS, contributing to the local control of pathogenic microorganisms during lactation. In addition to its immunological function, the XDH form plays a role in producing retinoic acid in mammary epithelial cells, with the potential to regulate cell differentiation and epithelial integrity [68].

5. Cathelicidins and Defensins as Key Antimicrobial Mediators of Mammary Gland Protection Induced by Neonatal-Lactation Interactions

Cathelicidins are evolutionarily conserved antimicrobial peptides, present in all vertebrates; humans only express LL-37 [69,70]. This 37 amino acid peptide (4.5 kDa, 37 amino acids + 6 charge) is stored as inactive hCAP-18 and activated through proteolytic cleavage by cellular proteases (proteinase 3, kallikrein) or microbial enzymes (aerolysin) during infection or tissue damage [71,72,73].

LL-37 exhibits membrane-directed antimicrobial activity thanks to its amphipathic properties, as it contains four lateral rings of phenylalanine, which provide essential physicochemical properties to interact with microbial membranes. Functionally, it is a “factotum” peptide, which also participates in tissue repair, wound healing, and the elimination of pathogens through the destabilization of cell membranes. LL-37 is synthesized by immune cells (neutrophils, monocytes, mast cells, dendritic cells), including platelets and skin and mucosal epithelial cells. It plays a fundamental role in innate immunity through direct microbial clearance and immunomodulation [70,73,74,75,76].

Suppose the expression of LL-37 remains low in adult skin under basal conditions [77,78]. In that case, newborns present significantly elevated levels regardless of inflammation, which suggests an improvement in the antimicrobial barrier that could compensate for the immunological immaturity of the neonate [70,73,74,75,76]. The active form of LL-37 is found mainly in maternal milk, secreted by breast tissue cells. Until it closes, the mechanism of how this peptide process is carried out in the mammary gland has not been described, which raises the hypothesis that it could be induced by contact with the saliva of the newborn [77,78].

The proposed mechanism is supported by ultrasound evidence that demonstrates the retrograde flow of infant saliva through the breast ducts during suction [67]. The feedback of the newborn’s saliva, rich in epithelial cells and active proteins (elastase, trypsin, kallikrein) and other enzymes derived from the oral epithelium, could be responsible for the proteolytic activation of hCAP-18 to LL-37 in the breast tissue. This process of proteolytic activation would allow a dynamic adaptive response of the maternal breast to the oral microbiome of the infant, adjusting the production of antimicrobial peptides according to immediate immunological needs [79]. These studies propose the idea that infants actively contribute to the homeostasis of the breast microenvironment, transforming themselves from passive receptors of immunity to dynamic regulators. Therefore, the infant’s saliva in the oral cavity receives maternal immune factors, which also transmit biochemical signals that reconfigure mammary immunity, exemplifying a true immunological symbiosis. The multifunctional activity of LL-37 and its ability to regulate cellular migration and angiogenesis allow double protection of breast tissue, preventing infectious processes such as mastitis and contributing to its regeneration after mechanical damage induced by frequent suction. The capacity of LL-37 to regulate cell migration and angiogenesis provides double protection for the breast tissue [70,73]. It demonstrates that the newborn acts as both a receptor and a catalyst for the local maternal immune response. These consulted references reveal multiple opportunities for future research. For example, it would be possible to evaluate the activation of hCAP-18 to LL-37 induced by infant saliva using mammary epithelial cell cultures, proteomic analysis, and immunoabsorbent assays to quantify the activated peptide. Furthermore, based on the functions of antimicrobial peptides, in vitro chemotactic assays can be used to characterize the immunomodulatory effects of LL-37 and defenses on epithelial and immune cells present in the mammary gland, as well as other complementary techniques that allow these hypotheses to be validated.

Breastfeeding also induces an immune response mediated by antimicrobial peptides called defensins, peptides rich in cysteine with conserved β-lamin structures, classified as α-defensins or β-defensins. The α-defensins (HNP1–4) are found to be enriched in neutrophils, whereas HD5 and HD6 are confined to Paneth cells [80,81,82,83]. β-Defensin 1 is expressed in epithelial barriers, such as the respiratory, urinary, gastrointestinal, and mammary mucosa. The basal defense of beta-defensin 1 (HBD-1), whose active production is confirmed in 95% of milk cells, increases drastically at the beginning of lactation [84,85]. The ELISA analysis revealed that the callus is exceptionally rich in α-defensin HNP-1 (≈33.0 µg/mL) and β-defensin HBD-2 (≈31.3 µg/mL). Indeed, HBD-2 levels are significantly higher in colostrum (median 8.52 µg/mL) than in mature milk (0.97 µg/mL) [86,87]. These high concentrations directly protect the maternal tissue through a double action: an antimicrobial effect that alters bacterial membranes and a chemotactic function that recruits immune cells such as neutrophils, monocytes, and lymphocytes at the site [84,88,89]. This combination demonstrates that defenses are essential for innate immunity, protecting the breast from infections during its most vulnerable phase.

6. The Transforming Role of Fetal Cells in Maternal Mammary Protection and Regeneration

The mother-child connection manifests before birth through fetal microchimerism: a transfer of fetal cells that persist in the mother’s body. Although the placenta was thought to be a barrier, molecular biology techniques, such as Polymerase Chain Reaction (PCR) to detect Y chromosome sequences, have shown that this cell trafficking is a common and long-lasting event, with fetal DNA detectable in the mother up to 43 years after birth [90]. Fetal microchimerism involves a remarkable transfer of fetal cells that reside and persist in the mother’s body; there is a cellular exchange in which fetal cells (such as trophoblasts and CD34+ stem cells) cross the placental barrier during pregnancy, either through microtrauma or molecular adhesion mechanisms (ICAM-1/VCAM-1) (Figure 3). These cells can persist in the mother for decades and play a dual role, promoting tissue repair and even in cancer [91]. A study of 179 women (91 with ductal carcinoma in situ and 88 controls) revealed that 85% of healthy women had detectable male fetal cells, compared to 64% of cancer patients (OR: 0.26; CI: 0.12–0.56). The protective effect was greater in women with multiple children and their first birth before age 30, acting as a “natural microtransplant” that eliminates premalignant cells through immunosurveillance. However, they may also increase the risk of autoimmune diseases [92]. Studies show that between 43% and 56% of healthy women present fetal microchimerism, compared to only 14% and 26% in cancer patients (OR: 0.23–0.29), with this effect being more marked in multiparous women. Fetal cells persist in breast tissue, acting as immunological surveillance mechanisms against tumors, suggesting therapeutic potential [93].

This evidence of protective fetal microchimerism indicates that the maternal breast is not simply a passive organ for milk supply, but a dynamic tissue formed by maternal and fetal cellular contributions, where fetal cells make a difference in protection against breast cancer. In this context, fetal microchimeric cells may act as sentinel and reparative elements, promoting epithelial regeneration, maintaining ductal integrity, and modulating local inflammatory responses. The potential of fetal cells to repair or protect breast tissue during lactation invites a paradigm shift in our perspective on mother–infant cellular interactions as both immunologically and structurally restorative.

7. The Favorable Impact of Breastfeeding Through Its Infant-Induced Microbiome

Breast milk provides microbiota that constitutes a fundamental initial ecological inheritance for the newborn. Species such as Staphylococcus, Streptococcus, Bifidobacterium, and Lactobacillus colonize the newborn’s intestine and share remarkable genetic similarities with strains present in infant milk and feces [94,95]. Until recently, this transmission was assumed to be unidirectional: from the breast to the infant. However, the newborn may also play an active role in shaping the mammary microbiota: through bacterial retrotransfer during suckling, oral microorganisms ascend to the mammary ducts, inducing local immune responses and modulating the mammary microenvironment (Figure 4) [96,97,98].

Some bacteria in breast milk can be transferred from oral and skin bacteria, which enter the mammary ducts during suckling in a process called retrograde flow [67]. The human breast milk (HBM) bacteriome is crucial in mastitis, a painful inflammatory disease that affects approximately 30% of lactating women [99,100]. An imbalance in this mammary microbial community is a key predisposing factor for the development of mastitis, often leading to premature cessation of lactation. Recent research highlights that specific probiotic strains, such as Lactobacillus salivarius and Lactobacillus gasseri, isolated directly from breast milk, are emerging as an effective and more natural alternative for managing and preventing mastitis. In support of this, a randomized clinical trial demonstrated that daily oral administration of 9 log10 CFU of Lactobacillus fermentum CECT5716 or Lactobacillus salivarius CECT5713 strains, also isolated from breast milk, for three weeks significantly reduced milk bacterial counts and clinically improved infectious mastitis in lactating women, achieving lower recurrence rates compared to standard antibiotic therapy [101].

Regarding the human breast milk (HBM) microbiome, HBM-derived Enterococcus faecalis and Staphylococcus hominis strains have been shown in vitro to exert selective antiproliferative and apoptosis-inducing activity in breast cancer cells (MCF-7) without showing significant cytotoxicity in normal breast cells [102].

This two-way microbial dialogue exerts a potential protective effect against mastitis: by promoting an environment dominated by commensals such as Streptococcus salivarius, which inhibits pathogens such as Staphylococcus aureus, the defenses of the mammary tissue are reinforced [103,104]. Intriguingly, this protective microbial exchange represents just one dimension of a far more complex symbiotic relationship. Beyond microbial transfer with the infant, emerging research reveals that the mammary gland’s own epithelial cells undergo profound transformation mediated by resident microbiota.

Recent molecular evidence shows that mammary epithelial cells themselves undergo measurable changes mediated by resident microbiota. Arnone et al. demonstrated that endocrine-targeting therapies such as tamoxifen shift mammary microbiota composition, increasing Lactobacillus spp. and other commensals in both murine and primate models. Intra-nipple administration of probiotic strains (Lactobacillus casei, L. plantarum, Bifidobacterium lactis) not only modified the ductal microbiome but also induced transcriptional reprogramming of more than 600 genes related to glycolysis and energy metabolism, increased tricarboxylic acid cycle intermediates, and reduced lactate accumulation in mammary tissue. Significantly, these microbial shifts correlated with decreased tumor cell proliferation (lower Ki67 expression) and improved tumor-free survival in experimental models. In human ER+ tumors, the abundance of LTA-positive bacteria was inversely associated with proliferation indices. These findings provide direct evidence that mammary cells respond to microbial cues, reinforcing the concept of a bidirectional infant–maternal symbiosis that extends to cellular and metabolic regulation within the breast [105]. While these mechanistic insights derive primarily from animal models and in vitro studies—understandably given the profound ethical and technical challenges of conducting such invasive microbiological and molecular research in lactating women—they nevertheless reveal fascinating possibilities for mammary gland regulation that warrant further cautious investigation in human contexts.

8. Critical Discussion, Limits and Future Directions

The present synthesis advances a novel perspective in lactation biology: the mother–infant dyad operates as a bidirectional immunological and metabolic unit, where the neonate contributes not only to its own nutrition but also to the protection of maternal mammary health. Among the proposed mechanisms, the neuroendocrine reflex elicited by suckling is supported by the most substantial body of direct human evidence. Numerous studies have measured maternal oxytocin and prolactin surges during breastfeeding and established their downstream effects on stress reduction, immune modulation, and milk ejection [35]. This mechanism, therefore, stands as the most robustly documented infant-driven contribution.

In contrast, other mechanisms—namely the enzymatic activation of the XO/LPO system by infant saliva, the proteolytic activation of antimicrobial peptides, retrograde microbial transfer, and the role of fetal microchimerism—rest on indirect or hypothesis-generating evidence. For instance, ex vivo experiments elegantly demonstrate that saliva–milk mixing produces antimicrobial H₂O₂ [55,61], and that hCAP-18 can be proteolytically converted to LL-37 [71]. Similarly, metagenomic studies reveal shared microbial strains between the infant’s mouth and breastmilk [103] and clinical trials with probiotics indicate that ductal microbiota modulation influences mastitis outcomes [101]. Meanwhile, microchimerism is well-documented by molecular detection of fetal cells in maternal tissues [91], though direct functional proof in the lactating breast remains lacking. Collectively, these data delineate a biologically coherent framework but highlight the need for temporally resolved, in vivo confirmation during breastfeeding (

Table 1. Synthesis and Levels of Evidence for Infant-Driven Protective Mechanisms in the Lactating Breast.

Mechanism & Level of Evidence	Claimed Protective Effect in the Maternal Breast	Molecular Mediators and Justification with Key References
Hormonal modulation Direct Human in vivo [31,33,34,35,36,37,43]	Reduced stress and inflammation; enhanced immune surveillance; milk ejection reflex	Neonatal suckling triggers maternal oxytocin (OXT) and prolactin (PRL) release, measured directly in lactating women during feeding. These pulses modulate immune cells expressing OXTR, dampen stress responses, and promote lactation efficiency. Although direct links to reduced mastitis incidence remain associative, the neuroendocrine reflex itself is robustly documented in humans.
Enzymatic activation XO/LPO system Indirect (Human/Model) [55,56,57,58,59,60,61,62,63,64,65,66]	Local antimicrobial activity via H2O2/OSCN− production in the ductal space	Human milk contains xanthine oxidase (XO) and lactoperoxidase (LPO). In vitro mixing of infant saliva (rich in hypoxanthine/xanthine) with milk produces physiologically relevant H2O2 that inhibits pathogens. While ductal in vivo measurements during suckling are lacking, the biochemical plausibility and ex vivo human demonstrations justify this indirect level.
AMP activation (hCAP-18 → LL-37, β-defensins) Plausible Hypothesis [70,71,72,73,74,75,76,77,78,79,84,85,86,87,88,89]	Direct antimicrobial effect, immune cell recruitment, tissue repair	Milk contains hCAP-18, which can be processed by proteases (proteinase 3, kallikreins) into the active peptide LL-37. In vitro studies confirm this conversion. Neonatal saliva contains proteases, suggesting a plausible pathway for local activation. However, direct in vivo evidence of infant saliva proteases activating AMPs within ducts during nursing is absent, so this remains a biologically well-grounded hypothesis.
Fetal microchimerism Indirect (Human/Model) [91,92,93]	Long-term immune surveillance, tissue repair, possible tumor suppression	Fetal cells persist in maternal tissues for decades. Studies detect microchimeric CD34+ progenitors in breast tissue and blood. Epidemiological data suggest associations with reduced breast malignancy. However, direct demonstrations that these cells actively protect lactating ducts against infection or inflammation are lacking; thus, evidence is indirect and correlative.
Microbial transfer (retrograde flow) Indirect (Human/Model) [67,96,97,98,99,100,101,102,103,104]	Colonization of ducts with infant oral commensals; competitive exclusion of pathogens; immune modulation	Strain-level metagenomics show shared bacteria between infant oral cavity and breastmilk, demonstrating that maternal probiotic administration modifies mastitis outcomes, supporting ductal microbial modulation. While real-time in vivo proof of infant-to-breast retrograde seeding remains elusive, correlative human data and probiotic intervention studies provide indirect evidence.

Abbreviations: OXT, oxytocin; PRL, prolactin; XO, xanthine oxidase; LPO, lactoperoxidase; H₂O₂, hydrogen peroxide; OSCN⁻, hypothiocyanite; AMP, antimicrobial peptide; hCAP-18, human cationic antimicrobial protein-18; LL-37, cathelicidin peptide; OXTR, oxytocin receptor.

).

Key limitations arise from the difficulty of sampling the mammary ductal environment during active nursing. Biochemical mediators such as H₂O₂ or LL-37 are unstable, and ethical constraints limit invasive procedures in lactating women. Moreover, causality remains unproven in most cases, as associations could be confounded by maternal variables such as diet, antibiotic use, or mode of feeding (direct breastfeeding versus pumping). High interindividual variability further complicates generalization.

A further significant limitation is the extrapolation of findings from animal models. For example, while studies on bovine epithelial cells offer insights into mammary immunity, their direct relevance to human lactation must be critically evaluated. There are crucial physiological and molecular differences between the bovine udder and the human breast, including variations in hormonal regulation, milk protein synthesis pathways, and the composition of immune cells present in milk [106]. Furthermore, single-cell transcriptomic analyses have revealed human-specific transcriptional programs active during lactation that are not mirrored in animal models, underscoring the necessity of human-focused research [107].

Future progress requires innovative experimental designs that can bridge this translational gap and reduce the reliance on animal studies. Priority should be given to (i) minimally invasive ductal sampling before and after feeds to capture transient biochemical changes, (ii) strain-level metagenomics in dyads with precise temporal resolution, and (iii) in situ imaging of retrograde flow using safe tracers. Crucially, the development and application of advanced human-relevant in vitro models is paramount. Human breast organoid systems, derived from breast milk cells, now allow for mechanistic studies of lactation in a physiologically relevant, three-dimensional context [107,108]. These organoid models, alongside other mammary epithelial cell culture systems, provide powerful tools to investigate milk secretion, epithelial function, and mother–infant interactions at the molecular level, something previously difficult to achieve [109,110].

Such approaches would upgrade current hypotheses from indirect to direct human evidence and permit rigorous testing of causality. From a translational perspective, confirmation of these mechanisms could revolutionize mastitis prevention and treatment by shifting emphasis toward sustaining infant-driven protective pathways. Supporting direct breastfeeding, fostering ductal microbial resilience through probiotics, and monitoring maternal–infant microbiome synchrony could all become clinically actionable. On a broader scale, microchimerism and long-term maternal immune modulation suggest that lactation may have implications extending into chronic disease prevention and regenerative biology.

In summary, while the oxytocin–prolactin axis provides solid direct evidence of infant-driven maternal protection, other proposed pathways remain promising but require decisive in vivo demonstration. Addressing these knowledge gaps, including the careful validation and contextualization of findings from animal models by using human-specific systems like organoids, through targeted experimental designs, will be critical to transform plausible hypotheses into established biology and to harness these insights for maternal health, which has not been favored by maintaining a unilateral idea.