Infant immune systems set on low to encourage microbiome growth

This newborn mouse is building its microbiome!

Credit: Wikimedia Commons

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.