By Hilary Metcalfe
When I was breastfeeding, one of the most mind-blowing realisations was this: my gut was training my baby’s immune system.
Every bite of food I ate, every herb I infused, every fibre I digested wasn’t just for me, it was setting up my daughter’s defences. Because breastmilk is more than calories. It’s an immunological blueprint, shaped by my digestion and my diet, passed gland-to-gut-to-baby in every single feed.
This is why I became obsessed with the science of milk as immune therapy.
The Immunological Power of Milk
Breastmilk isn’t passive. It’s immunologically alive.
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Secretory IgA (sIgA): coats the baby’s gut lining, neutralising viruses and bacteria before they can colonise.
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Lactoferrin: an iron-binding protein that starves pathogens and directly modulates inflammation.
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Human Milk Oligosaccharides (HMOs): indigestible sugars that feed the infant’s gut flora, encouraging growth of Bifidobacteria while blocking harmful microbes.
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Immune Cells and Cytokines: T and B lymphocytes, plus signalling molecules, are transferred directly from mum to baby (Demers-Mathieu et al., 2019).
This is adaptive immunity in action. But here’s the kicker: much of it depends on maternal digestion.
The Entero-Mammary Pathway: Gut to Gland
One of the most fascinating pathways in lactation biology is the entero-mammary link. Immune cells in the gut-associated lymphoid tissue (GALT) “sample” microbes and antigens from mum’s digestive tract. They then migrate to the mammary glands, where they secrete targeted antibodies, especially IgA, into breastmilk.
Translation? If I ate yoghurt, or was exposed to a seasonal virus, my immune system processed that through my gut, then passed custom-made antibodies to my baby via milk. It’s a living feedback loop: mum’s gut experience → baby’s immune education.
The Microbiome Connection
It’s not just immune cells. The maternal microbiome also leaves fingerprints on milk.
Studies show that a diverse, fibre-rich maternal diet increases microbial diversity in both gut and milk (Cortes-Macías et al., 2021). Beneficial strains like Bifidobacteria and Lactobacillus appear in breastmilk, seeding the infant gut and training the developing immune system.
This means the plant foods, prebiotics, and polyphenols I chose for myself weren’t just “good habits.” They were shaping the bacteria that would colonise my baby’s intestines, and by extension, her immunity for years to come.
Nutrients as Immune Cofactors
Some immune factors in milk are relatively diet-independent (like lactose). But others swing widely with maternal intake and absorption:
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Vitamin A: key for mucosal immunity and antibody production; deficiency reduces IgA transfer (Allen, 2012).
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Vitamin C: improves immune cell function and supports iron absorption.
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Zinc & Selenium: required for immune enzyme activity.
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Polyphenols: plant compounds with anti-inflammatory and antioxidant effects that influence immune tone in milk.
If your digestion is compromised, bloating, poor absorption, rushed meals, these cofactors don’t reach circulation efficiently. And if they don’t reach circulation, they don’t reach milk.
The Breastfeeding Herbata: Designed for the Gut–Immune–Milk Axis
This is exactly why I formulated Breastfeeding Herbata as part of my own postpartum ritual. I wanted a daily infusion that addressed two things at once: digestion and immunity.
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Fennel: eases gut motility and supplies phytoestrogens that support prolactin and IgA transfer.
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Moringa: high in vitamin A and antioxidants, boosting both micronutrient transfer and immune resilience.
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Nettle & Oat Straw: mineral-rich, replenishing iron and zinc, cofactors for immune enzymes.
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Tulsi & Vervain: adaptogens and nervines that reduce cortisol, supporting oxytocin and stress-buffered digestion.
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Cinnamon & Cardamom: improve circulation and stabilise post-meal blood sugar, smoothing nutrient absorption.
For me, sipping it wasn’t just hydration. It was an immune ritual, every cup a signal to my gut and glands: digest, absorb, protect.
When we talk about “breast is best,” we rarely explain why. The truth is: breastmilk is immune engineering in real time. It’s antibodies, microbes, micronutrients, and enzymes, all filtered through a mother’s gut, all customised by her diet.
The gut is the entry point. The mammary gland is the amplifier. Together, they write the blueprint of a baby’s first immune system.
That’s why what we eat and how we digest it matters. And why something as simple as a cup of Breastfeeding Herbata can feel profound: because it supports not just milk volume, but the invisible immune story we pass on to our children.
ABOUT HILARY
Hilary is the Co-Founder of the SABI, a Holistic Nutritionist, natural, whole foods Chef, product developer and advocate for women getting to know their bodies, cycles and selves better. Born in Los Angeles, California and raised in Baja California, Mexico, she now lives in Los Cabos with her partner Kees, a curly-tailed rescue dog from Curacao, Flint and her rainbow babies Paloma and Bea.
HORMONAL & PROUD
Created as a brand to help women navigate the toughest moments in pregnancy, childbirth, postpartum — and practically every stage of life –– The SABI is changing the narrative around our hormones from one of taboo, embarrassment, and loneliness to awareness and even, pride. As more than a wellness brand, The SABI offers a carefully-crafted line of products to carry you through your hormonal journey, including rituals, supportive tools, and ancient herbal remedies that have been tested time and time again by women and now come backed by medicine. The SABI is a blend of science and nature conceived by women who have experienced the joys and deep struggles of bringing a child into the world, the pains of a heavy, difficult period, miscarriage, and difficulty conceiving.
We invite you to get to know your body and its cycles better –– to really understand what is going on inside. Learn to use your hormones to your advantage no matter your stage of life, and know that you can support and balance your hormone levels. We are here to help with the information, understanding and natural tools to support your body and the emotional process along with it.
DISCLAIMER
The SABI blog and articles are not meant to instruct or advise on medical or health conditions, but to inform. The information and opinions presented here do not substitute professional medical advice or consultations with healthcare professionals for your unique situation.
References
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Field CJ. The immunological components of human milk. J Nutr. 2005.
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Lönnerdal B. Nutritional roles of lactoferrin. Curr Opin Clin Nutr Metab Care. 2010.
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Bode L. Human milk oligosaccharides. Glycobiology. 2012.
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Demers-Mathieu V, et al. Immune cell transfer through human milk. Front Immunol. 2019.
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Cortes-Macías E, et al. Maternal diet shapes breast milk microbiota. J Dairy Sci. 2021.
Allen LH. Maternal micronutrient malnutrition. Am J Clin Nutr. 2012.
