Study reveals immunological mechanism involved in breastfeeding

A living fluid capable of carrying nutrients, beneficial bacteria, and specific immune cells to protect the baby against viruses and microorganisms in the family's environment. This is how science is beginning to decipher breast milk. A study conducted at the Salk Institute for Biological Studies in the United States, published in July 2025 in Nature Immunology , showed how immune system cells migrate from the mother's intestine to the mammary glands during breastfeeding.

"This means that breast milk carries 'tailored' antibodies against viruses and bacteria from the family's environment," explains nurse Natalia Turano, who works as a breastfeeding specialist at Einstein Hospital Israelita. She adds that, in practice, it's as if the baby receives personalized and immediate protection—something no formula can offer.

Most breastfeeding studies focus on the relationship between milk quantity and baby health. The Salk Institute's work took a different direction by unraveling how this migration occurs. By analyzing the cells, scientists found not only a higher concentration of T lymphocytes in breast tissue during lactation, but also something unprecedented: some of them originated in the intestine.

Pathway of defense cells

To reach these conclusions, the researchers observed mammary gland tissue from mice at different stages, from pre-lactation to post-lactation. They found that three specific T cell subtypes (CD4+, CD8αα+, and CD8αβ+) increased in number during lactation.

These defense cells inhabit mucosal tissues such as the intestines and lungs, precisely because they are areas more vulnerable to contact with external microorganisms. What was surprising was that the lymphocytes lined the mammary epithelium in the same way they would the intestine, even displaying "fingerprints" typical of cells residing in the intestinal tract.

Analysis of human tissue and milk samples, conducted in collaboration with the Human Milk Institute at the University of California, San Diego, confirmed the presence of these same cellular equivalents. The team then showed that the presence of microbes directly modulated the production of these cells: mice raised in germ-free environments had fewer intraepithelial lymphocytes in their mammary glands than those exposed to microorganisms.

Evidence shows that, during lactation, the mammary gland transforms into a mucous tissue prepared to interact with the environment and transfer an immunological barrier to the baby, influenced by the maternal microbiota. This adaptation reinforces the role of breastfeeding as a link between immunity and mother-child development.

"Furthermore, it reinforces the importance of exclusive breastfeeding, skin-to-skin contact, and maintaining breastfeeding even in the face of challenges," says Turano. "It also indicates that practices such as the rational use of antibiotics and encouraging balanced diets in the mother can positively impact the microbiota and, consequently, the baby's health."

Breastfeeding brings countless benefits

The conclusion reinforces the World Health Organization (WHO) recommendation: if all babies were exclusively breastfed until six months, approximately 820,000 infant deaths could be prevented each year. In Brazil, the Ministry of Health follows the same guideline and recommends that breast milk be the sole food until the sixth month and that breastfeeding continue, with other foods, until the child is two years old or older.

The benefits of breastfeeding are proven in numbers and extend beyond the immune system. According to the WHO and the United Nations Children's Fund (UNICEF), babies who are exclusively breastfed for six months have fewer episodes of diarrhea and respiratory infections and a lower risk of sudden infant death, obesity, and type 2 diabetes later in life.

Protection isn't limited to childhood. Long-term analyses, including a Brazilian cohort followed for 30 years , show that adults who are breastfed for longer periods have higher IQs, higher educational attainment, and better economic income.

"Science shows that breastfeeding has numerous benefits: it strengthens the baby's immune system, reduces the risk of infections, obesity, and chronic diseases, and promotes cognitive and emotional development. For the mother, it reduces the risk of breast and ovarian cancer, aids postpartum recovery, and promotes emotional bonding," says the Einstein nurse.

For the lactation specialist, the new research paves the way for new questions, such as understanding in more detail how this cellular transit is regulated, which specific types of cells reach the breast, and how this can be used to develop vaccines or precision treatments.

“The exact mechanisms of protection against some chronic diseases throughout life and how social, psychological and environmental factors influence the maintenance of breastfeeding,” he says.

Brazil in an advantageous position

In Turano's view, better understanding the mechanisms of breastfeeding raises two important debates for society. The first is direct support for families, with increased access to lactation consultants, human milk banks, and multidisciplinary monitoring services, recognizing that each mother and baby have unique needs.

The second is in the field of public policies: investing in evidence-based incentive campaigns, strengthening the network of support rooms and milk banks, expanding maternity leave, and developing health education strategies that combat contradictory information on social media.

In this sense, Brazil starts from a position of advantage. The country is internationally recognized for its network of human milk banks, the largest in the world, and for its implementation of the Baby-Friendly Hospital Initiative, launched in partnership with the WHO and UNICEF in the 1990s.

These policies helped raise exclusive breastfeeding rates for babies up to six months from 2% in the 1980s to over 45.8% in 2019, according to the National Study of Infant Feeding and Nutrition (ENANI) . Furthermore, unlike countries like the United States, where only a quarter of babies reach six months of age on exclusive breastfeeding , the practice in Brazil is close to the WHO's global target.

"In the Brazilian context, these discoveries gain even more relevance. The country is already a global benchmark. Incorporating scientific advances into these policies can solidify Brazil as a model for supporting breastfeeding, ensuring equity, diversity, and universal access," concludes the nurse.

Source: Einstein Agency

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