Researchers have long known that children with older siblings have seemingly more active immune systems, but the reasons why continued to elude them.
Now a new study from Denmark by way of the medical journal Allergy provides some insight into the mechanisms that benefit younger siblings.
The Danish researchers studied 571 1-month-old babies and collected mucus samples from the children, according to NPR. Babies whose mothers had previously been pregnant had "significantly higher levels of signal proteins associated with triggering immune response," helping their bodies react to foreign objects.
The researchers say that's significant because it doesn't involve the usual allergy response they thought would play a part in boosting young immune systems. The proteins involved in their more robust immune systems are separate from the "Type 2" proteins that prompt allergic reactions, NPR reported.
"We train our immune system in very early life," study co-author Susanne Brix Pedersen told NPR. "Being able to train it seems to protect us for later in life."
As with so many things in science, the answers give way to more questions. The study's authors said they believe pregnancy could alter chemistry within the womb for a finite amount of time -- the benefits decrease the longer the mother goes between pregnancies, they said.
But the results could be influenced by factors outside the womb as well. One theory is that siblings who are older -- but not too much older -- could be exposing their younger brothers and sisters to microbes. The presence of an older sibling who plays outside or on the floor could expose the newborns to more of those microbes than if the newborns were only children living in more clean, clinical conditions.
That line of thinking is similar to studies that caution against using too much antibacterial soap and hand cleaner, which could deprive the immune system of microbes it needs to stay healthy. Children get dirty easily, the researchers pointed out, and as the older siblings age and "embrace personal hygiene," they may pass along fewer immune system-strengthening microbes, Brix Pederson said.
The before birth versus after birth benefits have given scientists more avenues for potential research. Not everyone agrees that the boosted immune systems can be credited to microbes the children are exposed to after they're born.
"I'm more biased that it's in utero, happening before birth," Wilfried Karmaus, a professor of epidemiology at the University of Memphis School of Public Health, told NPR.
The Danish study supports earlier efforts to understand why younger children seem to have stronger immune systems. A 2011 study by Japanese scientists followed 13,000 children between 7 and 15 years old, and sought to understand why older siblings are at greater risk of hay fever and food allergies than their younger brothers and sisters, according to Readers' Digest.
And in a study by Northwestern University researchers looked at the benefits of exposing young children to germs early in life. Thom McDade, Ph.D., director of the university's Laboratory for Human Biology Research, told WebMD that the findings may seem counterintuitive, but could have profound implications for children as they grow up.
"We're moving beyond this idea that the immune system is just involved in allergies, autoimmune diseases, and asthma to think about its role in inflammation and other degenerative diseases," McDade told WebMD. "Microbial exposures early in life may be important ... to keep inflammation in check in adulthood."