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Babies are born with some immunity, but they do not have as many tools as adults to fight diseases. Exposure to pathogens helps build their immune repertoire, but it also leaves them vulnerable to infection because many immune defenses are still developing.

What kick-starts immune system development is a question a team of researchers from Canada and the U.S. decided to explore in a mouse study now published in the journal Science Translational Medicine.

Researchers investigated whether the first friendly bacteria to colonize a newborn's gut help prepare its immune system to fight life-threatening infections. They identified calprotectin, an antimicrobial protein complex that builds up in the intestines of healthy newborn mice after their first exposure to these microbes.

While adults produce calprotectin mainly in immune white blood cells (neutrophils), newborns get most of it from intestinal epithelial cells, which form the single layer of cells lining the gut wall.

The way gut microbes and gut lining cells interact at birth can help shape a baby's early immunity. When early colonization by good gut bacteria was disrupted, calprotectin levels fell, leaving the mouse pups with an insufficient protective response. As a result, the pups became more vulnerable to sepsis and, in some cases, died after infection.

Among neonates, infections rank alongside premature birth and birth complications as the leading causes of death. Together, these causes account for almost four in every 10 deaths in children under 5 years of age.

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