Breastfeeding and immunity: what impact does breast milk have?

Breast milk protects against the adhesion and multiplication of pathogenic germs. Immunoglobulin is an antibody produced at mucosal surfaces (intestine) that neutralises toxins, viruses and pathogenic bacteria.

The production of IgA antibodies (Immunoglobulin A) in infants is still very immature. This is why their presence in breast milk is particularly valuable (referred to as exogenous supply) [2]. This ensures the transitional period until the child is able to produce their own. Antibodies line the intestinal wall, making it less permeable to germs. They form a barrier preventing most pathogens from binding to the cells of the mucous membranes and epidermis.

The protein known as lactoferrin can bind to two iron atoms. Since several pathogenic bacteria feed on iron, lactoferrin limits their growth by making iron unavailable. Lactoferrin is also found in colostrum. 

Some help prevent germs from attaching to the respiratory mucous membranes, boost key proteins in the immune defences (particularly to combat inflammation via cytokines), and destroy pathogenic germs (breast milk is also rich in immune cells such as macrophages, which phagocytose bacteria).

The role of the microbiota is critical, as it enables the body to distinguish beneficial bacteria from pathogenic ones.

A baby's digestive tract acquires its intestinal flora from the very first minutes of life in the case of a vaginal birth. To find out more on this topic, have a look at our article on the infant microbiome. 

After this initial colonisation, skin-to-skin contact after birth also allows for a first cutaneous encounter with maternal bacteria — contact that is renewed with each feed if the baby is breastfed. Furthermore, breastfeeding shapes the intestinal microbiota in early life, both directly through the infant's exposure to the milk microbiota and indirectly through factors in breast milk that affect bacterial growth and metabolism [3].

Breast milk contains prebiotics and probiotics, including oligosaccharides, the best known of which are Human Milk Oligosaccharides, which stimulate beneficial colonisation and reduce colonisation by pathogens [4].

The hallmark of (healthy) breastfed infants is the presence of large quantities of Bifidobacteria in their microbiota. These bacteria come from breast milk and persist all the way to the infant's gut.

Bifidobacteria play multiple roles: they produce organic acids and prevent the growth of pathogenic bacteria (through a competitive effect). Studies have also shown a strong interaction between the microbiota and TGF-beta, which play a role in regulating immune responses. Bifidobacteria appear to activate the production of TGF-beta.

The longer breastfeeding continues, the greater the effects on the microbiota. The WHO therefore recommends breastfeeding up to two years of age.

"The microbiota is said to be "resilient", meaning that at around 2–3 years of age, it will not change much for the rest of life. Medication will damage it to varying degrees. It will always try to return to its initial microbiota — that is, the one it had at age 2–3. Hence the importance of having a healthy microbiota at 2 years old." (Lynda, IBCLC consultant)

Breastfeeding is the best way to protect a baby from infections. In breastfed babies, gastroenteritis is less severe and shorter in duration [5]. There are reportedly 3 times fewer cases of serious diarrhoea than in bottle-fed babies, a 4 times lower risk of meningitis, and a 10 times lower risk of hospitalisation for a severe bacterial infection.

La Leche League International has also advised continuing breastfeeding, even when the mother is infected with a respiratory virus.

Indeed, the infant will already have been exposed to the virus through the mother or a member of the family, and continuing to breastfeed therefore carries more benefits than risks. Breast milk contains immune components that can protect the baby from the virus.

La Leche League also highlights the risks of changes to breastfeeding:
- emotional distress for both the baby and the mother, and an increased risk of the child becoming unwell without the immune protection provided by breastfeeding (in the event of sudden cessation)
- breastfeeding difficulties and a future breast refusal if a bottle is introduced
- a reduction in immunity (as expressed milk does not correspond exactly to the specific needs at any given moment)

Of course, if it is necessary to stop breastfeeding for medical reasons, to care for the mother, in those cases expressing milk is always recommended.

It is possible to be vaccinated while breastfeeding. Breast milk may even pass protective factors on to the baby. To find out more, have a look at our article on Covid-19 and breastfeeding.