Human breast milk was once considered sterile. Recent studies show that human breast milk is not a sterile substance and is considered an alive ecosystem. The breastmilk ecosystem comprises bacteria, live viruses, fungi, yeast, and minor genera. This ecosystem is called the human milk microbiome or HMM. (Consales 2022)

Regarding breastfeeding and its benefits, it is best to start an hour after the baby is born. This is because colostrum (which is milk that is produced in the first three to five days after birth) contains higher concentrations of immunoglobulins and immune cells. Studies show that milk lymphocytes are absorbed by breastfeeding infants, with results showing a transfer of immunological information that enhances an infant’s early immune responses. These studies also show that major histocompatibility complex or MHC (which is a group of genes that code for proteins found on the surfaces of cells that help the immune system recognize foreign substances) that suggest breastmilk leukocytes could be targeting infant Peyer’s patches or PP independently from MHC haplotypes. (Moles 2018) Stem cells or progenitors from these studies show that breastmilk transferred to the infant could play a role in immune maturation and regulation. These studies also show immune tolerance in maternal MHC antigens as the steam cells do not harbor MHC class one and class two antigens, mucosal ontogeny, and tissue repair and growth. (Moles 2018) These studies suggest that breastmilk cell trafficking could drive the neonatal immune system maturation and regulation response. 

Human milk is considered a symbiotic food. This is because human milk harbors safe bacteria that have the potential role of preventing neonatal infections and diseases. While the human milk microbiome may not be completely understood, studies suggest that the mammary glands possibly host the HMM microbiome or that it is a bystander that goes through constant influxes of microbes from exogenous sources. (Consales 2022) Studies demonstrate a comparison of women who gave birth via C-section and vaginally have vastly different bacterial diversity. That being women who gave birth vaginally have greater bacterial diversity and richness with higher levels of Bifidobacterium and lactobacillus spp than those who gave birth via C-section. Studies also show that maternal diet plays a role in the composition of HMM. The mother’s diet affects HMM composition more while pregnant than during lactation and breastfeeding. The mother’s mental state post-birth can also affect the HMM composition. If the mother experiences symptoms of stress, anxiety, or depression during the postnatal period, it can lower the human milk bacterial diversity. (Consales 2022) 

The neonatal immune system is a living complex with enhanced vulnerability to infections and diseases. Due to this, neonates need a fully equipped immune system with proper alert units to engage in when needed. (Yu 2018) These studies focus on the innate immunity of neonates and infants that emphasize the class of tissue residents, non-BCR, and non-TCR expressing lymphoid cells. The innate lymphoid cells or ILCs’ nomenclature is analogous to T calls and is divided into two groups of cytotoxic and helper-like cells. The ones comparable to cytotoxic T cells are represented by natural killer cells or NK cells, and the helper cells are typified by three subgroups of ILCs, ILC1s, ILC2s, and ILC3s based on their cytokine secretions and transcriptions. (Yu 2018)

Lactating mothers have to be on guard for microbial infections of the breast. This is because one-third of the premature cessation of breastfeeding is due to mastitis. The milk leukocytes perform a protective function. The leukocytes are haplo-mismatched with the infant MHC. This is how the infants avoid graft versus host attacks with cytotoxic T cells that make up three percent of the milk leukocytes. This is important to how the infant’s intestinal microbiome develops their T and B cell repertoires. It will also determine how the ILCs use other innate leukocytes. (Yu 2018) Human milk has about thirty-two non-cellular bioactive elements. These elements comprise carbohydrates, lipids, proteins, and about five cell types. Human milk also contains neutrophils, macrophages, lymphocytes, stem cells, epithelial cells, and microbes. Infants ingest about 100,000,000 maternal cells a day. Eighty percent of the cells are macrophages that come from the mother’s peripheral blood monocytes. Over ninety percent of these cells are viable. These bioactive components help protect the breast from infection and modulate the development and maturation of the neonatal immune system. (Yu 2018) 

The innate lymphoid cells occupy key intersections between adaptive and innate immunity. They function as foreign troops sent to vulnerable areas of the immune system to boost and train its defenses against unstable environments. The ILCs of human milk help shape the infant’s intestinal microbiome by modulating neonatal immunity. These milk ILCs will impart innate immunity in newborns and the mother. This is why it is important to protect the breast from microbial infections. Milk leukocytes will provide defense against infections for the mother and the baby. However, the mother must be vigilant and avoid infections as much as possible. (Consales 2022), (Moles 2018), and (Yu 2018)


Consales, A., Cerasani, J., Sorrentino, G., Morniroli, D., Colombo, L., Mosca, F., & Giannì, M. L. (2022). The hidden universe of human milk microbiome: origin, composition, determinants, role, and future perspectives. European journal of pediatrics181(5), 1811–1820.

Molès, J. P., Tuaillon, E., Kankasa, C., Bedin, A. S., Nagot, N., Marchant, A., McDermid, J. M., & Van de Perre, P. (2018). Breastmilk cell trafficking induces microchimerism-mediated immune system maturation in the infant. Pediatric allergy and immunology : official publication of the European Society of Pediatric Allergy and Immunology29(2), 133–143.

Yu, J. C., Khodadadi, H., Malik, A., Davidson, B., Salles, É. D. S. L., Bhatia, J., Hale, V. L., & Baban, B. (2018). Innate Immunity of Neonates and Infants. Frontiers in immunology9, 1759.

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  1. abstract-
    In her research she explains that recent studies have overturned the notion of human breast milk as sterile, revealing it as a thriving ecosystem known as the human milk microbiome (HMM). Comprising bacteria, viruses, fungi, and yeast, this ecosystem plays a vital role in infant health. Colostrum, the milk produced in the first few days after birth, is particularly rich in immunoglobulins and immune cells, enhancing early immune responses in newborns. She also states that studies suggest that breast milk lymphocytes and stem cells contribute to immune maturation and regulation, offering protection against infections. The composition of the HMM varies based on factors such as delivery mode, maternal diet, and mental state, impacting neonatal immunity.

    According to her research the neonatal immune system, vulnerable to infections, relies heavily on innate immunity, including innate lymphoid cells (ILCs). These cells, present in human breast milk, modulate neonatal immunity and shape the infant’s intestinal microbiome. Lactating mothers must guard against breast infections, as milk leukocytes protect against mastitis and contribute to the development of the neonatal immune system. Human breast milk contains various bioactive elements, including neutrophils, macrophages, lymphocytes, stem cells, epithelial cells, and microbes, which safeguard against infection and promote immune system maturation.

    Overall, human breast milk is not only a source of nutrition but also a vital contributor to infant health, offering protection against infections and supporting immune system development. Understanding the role of the HMM and milk leukocytes underscores the importance of breastfeeding and maternal health in ensuring optimal infant immunity.

    Jadynn Heineken

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