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.
Seeking new ways to better understand this process, Sing Sing Way, a pediatric infectious disease doctor at Cincinnati Children's Hospital Medical Center, wondered whether transferring immune cells from adults might rev up their immune systems. Yet when he and his colleagues injected infection-fighting cells from the spleens of adult mice into 6-day-old pups, nothing happened: The pups were just as vulnerable to harmful bacteria as control animals. Probing more deeply into this surprise, they found that the injected cells simply stopped functioning in the newborn animals. Then Way's group did the reverse transplant - they gave adult mice newborn immune cells that were inactivated in the pups and found that they "turned on" in the mature animals. These experiments "told us it wasn't a problem with the neonatal cells themselves," Way says. Rather, he believes, the environment - either a newborn body, or an adult one - guided how the cells behaved.