From the moment we're born, our body develops a delicate protective coating made entirely of microbes.
Knowing which bacteria, viruses, and yeast to keep, and which to reject, is a complex process that can make the difference between a healthy immune system and life-long allergies and auto immune disorders.
Just how we manage this system isn't entirely clear. As best as scientists can work out, mechanisms that protect our bodies from our mother's body continue to serve a role in the diplomatic relations with microbes in our first weeks outside the womb.
A recent study by researchers from the University of Birmingham in the UK has now provided new insights into the process, finding that mother's milk promotes the growth of important immune cells that help manage inflammation.
"The influence of the type of milk received on the development of the immune response has not previously been studied in the first few weeks of life," says neonatologist Gergely Toldi from the University of Birmingham.
"Prior to our research, the outstanding importance and the early involvement of this specific cell type in breastfed babies was unknown."
The study looked at the immune functions of 38 full-term newborns, all delivered via caesarean section. Infants were distinguished by whether they were breastfed, formula-fed, or received a mixture of both, and had blood and stool samples taken at birth and again three weeks later.
The difference between the two groups' regulatory T-cells – a type of white blood cell that helps keep a lid on immune responses – was profound, with those who had been exclusively breastfed seeing the cells double in number by three weeks, compared with those who were exclusively formula-fed.
The T-cells themselves also showed greater expression of a surface marker linked with increased suppressive activity. Their activity also seemed especially geared to acting specifically against the mother's body.
But the process of breastfeeding is itself far from sterile, transferring not just the mother's skin microflora, but many of her own cells. In those precarious early weeks, the barrage of foreign material could easily overwhelm a child's naive immune system.
Boosting regulatory T-cells to protect against mum's stray cells could be just the thing to keep infants healthy.
Exactly why milk has this effect isn't entirely clear. A recent study in mice indicates a feedback loop is at work, with a biochemical collaboration between mother and child fine-tuning the process.
Still, the exact mechanisms will need to be teased apart in future research.
An observation made by the researchers in this latest study could provide a clue. It was also noted that specific gut bacteria known to support regulatory T-cell function were more abundant in the digestive system of the breastfed newborns, suggesting their enhanced growth could be responsible.
The decision to focus on infants only delivered by caesarean was a deliberate one, to keep immune variables down. But how vaginally delivered children compare is another avenue for further study, especially given differences in acquisition of microflora between delivery methods.
It's important to note that in today's world, 'breast is best' is an added pressure many mothers don't need. Formula feeding can often be a reluctant but practical decision, either to reduce stress or, in many cases, simply because there are few other choices.
Replicating the inherent benefits of breastfeeding, if that is indeed possible, depends largely on studies like these, which go a long way to exploring where those benefits could come from.
But there's no doubt that in a direct comparison of infant feeding methods, mother's milk straight from the breast continues to give newborns every advantage, with minimal cost to the environment to boot.
"We hope this invaluable new insight will lead to an increase in rates of breastfeeding and will see more babies benefit from the advantages of receiving breastmilk," says Toldi.
"Furthermore, we hope for those babies who are formula fed, these results will contribute to optimising the composition of formula milk in order to exploit these immunological mechanisms."
This research was published in Allergy.