Doctors and scientists have long known that infants have fragile immune systems. They lack a robust defense system against invading pathogens, developing one as they age. In general, this suggested that the infant immune system simply needs time to get stronger. But new research indicates that infants' immune response is actively being held down by immunosuppressive cells.

Some precursors to red blood cells are marked by the protein CD71. Doctors at the Children's Hospital in Cincinnati found that, in human umbilical cord blood and newborn mice, these cells make an enzyme called arginase-2 that actively inhibits the immune response. When they transferred immune cells from newborns into adult mice, they found that the cells themselves were fully functional and capable of ramping up an immune response in the adult environment. This shows that the key is how the immune cells are regulated, not whether the infant immune cells themselves are immature.

Why would infants be intentionally susceptible to infection? The answer comes from one of the hottest new fields in medical research—the microbiome.

The microbiome is science shorthand for all of the benign and beneficial microbes that live in our bodies, helping us digest food and keeping populations of pathogens in check so we don't get sick. In recent years, thanks to high-throughput DNA sequencing, scientists have learned just how vast and varied the microbes who call us home can be.

The several pounds of bacteria that you carry around every day play a complex role in your health, and ongoing research continues to explore the ways that the make-up of our microbiome might effect our susceptibility to everything from diabetes to cancer.

But babies are born without a microbiome because they've spent the past nine months in the sterile womb and their digestive system is a blank slate. The first order of business in life, literally, is to start collecting one's bacterial community—babies swallow some of the microbes present along the vaginal canal, and that becomes the beginning of their microbiome. This isn't just an accident; research has shown that pregnant women's bodies actually change so that they host different types of bacteria in their vaginas. This may be an evolutionary adaptation to give the infant the best biome possible. Babies born through cesarean section miss out on establishing some of those key species according to research.

For their part, the infants seen to be tamping down their immune response to allow the beneficial bacteria to establish themselves. Obviously, this also makes them more susceptible to infection from dangerous bacteria as well, so developing a healthy microbiome is a bit of a tradeoff. The authors of the Cincinnati hospital study call it "unfortunate by-product of the greater benefits of active suppression during this crucial developmental period, when tolerance to commensal microbes is more uniformly advantageous."

In their paper, published this week in Nature, the authors also write that they expect that this finding could spur further research on the mechanism for infant immunosuppression. The goal would be to work around it to protect infant's health in the short term while they are busy building a healthy mircobiome—which will hopefully keep them healthy in the long term.

Nature, November 2013. DOI: doi:10.1038/nature12675