Why newborn's immune systems aren’t fully functional

One of newborns’ biggest vulnerabilities is largely invisible: In the weeks after birth, babies are especially susceptible to infection because their immune systems aren’t fully functional. There are a handful of theories to explain this liability, and now a research team has added a new one to the list: Immune suppression in early life might help prevent inflammation in the infants’ intestines as they become colonized by the helpful bacteria they need to stay healthy.

Newborns are more likely than older babies to catch, and die from, serious infections. The reason is fuzzy—indeed, there may be more than one explanation. One theory is that much like their brains, their lungs, and the rest of their bodies, infants’ immune systems just haven’t fully matured yet. Another is that both mothers-to-be and their in utero companions have suppressed immune systems, so that neither rejects the other. After birth, the thinking goes, it takes babies a month or so to boost their immunity.

Others in Way’s lab study the gut microbiome, the constellation of healthy bacteria that populates our intestines. Newborn mice, just like human babies, are born “clean,” with little intestinal bacteria. Very rapidly that changes. Way wondered whether there might be some connection between this colonization and what looked like a purposeful suppression of the immune system in his mice.

To find out, his group focused on immune cells that eventually develop into red blood cells and that express a surface receptor called CD71, which causes immune suppression of other cells. Knocking out about 60% of these CD71 cells—as many as their technology could manage—was followed by significant inflammation in the intestines of the mouse pups. Way and his colleagues also found that, as the mice grew, fewer and fewer cells boasted CD71 receptors, suggesting the suppression wasn’t needed. He theorizes that that’s because the gut has been colonized by that point.

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