Researchers have noted a positive relationship between older siblings and allergies since at least 1989, when a study following British children for 23 years found that the more older siblings a child has, the less likely she or he will be allergic to airborne particles like dust and pollen. But exactly how older siblings boost younger sibling health has continued to flummox scientists.
To learn more about the mechanism behind this resilience, Danish researchers studied 571 1-month-old babies. Like many things concerning small children, the study involved snot — the researchers collected samples from each infant’s nose. They found that infants whose mothers had been pregnant before had significantly higher levels of signal proteins associated with triggering immune response.
This immune “signature” may make younger siblings’ systems “more alert” to possible sources of infection, according to systems biologist Susanne Brix Pedersen of the Technical University of Denmark, who co-authored the paper. While these proteins cause cells to react to foreign microorganisms, the proteins are not the “Type 2” immune chemicals that help trigger allergic reactions. The researchers think younger siblings may therefore be primed to respond to foreign objects like pollen through means other than allergy.
“We train our immune system in very early life,” says Brix Pedersen. “Being able to train it seems to protect us for later in life.”
The researchers are not sure if older siblings benefit babies before birth, after, or both. Brix Pedersen says that after a first pregnancy, a mother’s immune system may recognize another fetus from the same father, and therefore treat it differently, potentially leading to changes in the way subsequent babies’ immune systems develop.
In the Danish study, the researchers found that the more time that had passed between pregnancies, the lower the levels of helpful immune proteins younger siblings had. This decreased benefit over time could suggest that previous pregnancies change the chemistry inside the womb and that these changes wear off with time.
On the other hand, Brix Pedersen points out that the pattern could also be explained by post-birth benefits. Older siblings may be at their dirtiest when young, and therefore the most helpful in exposing younger siblings to microbes and building up a stronger immune response. Once older siblings age and embrace personal hygiene, they may bolster babies’ immune systems less. The researchers intend to continue studying the infants as they age to see if their immune signatures do indeed protect them against asthma and allergy as expected.
“I’m more biased that it’s in utero, happening before birth,” says Dr. Wilfried Karmaus, a professor of epidemiology at the University of Memphis School of Public Health who was not involved in the study. Citing the impact of birth order on obesity and diabetes, Karmaus says, “We have to consider how to apply this knowledge to prevent allergies. How can we mimic chemistry to make it as though the first pregnancy is the second pregnancy?”
Older siblings may get the raw end of the immune system deal, but older sibling Morgan Rees, who was not involved in the study, isn’t bitter. Rees, a 19-year-old rising sophomore at the University of Pennsylvania, says she doesn’t begrudge her younger brother his health, though she has asthma and he does not.
“My brother and I are both cross-country runners,” says Rees, whose asthma is exercise-induced. After she realized she had asthma in high school, she says she kept running the 1-mile event in track, but her times did not improve, and her asthma was “super disruptive.” Still, she enjoys cheering on her brother, who is now a senior in high school and asthma-free.
“I helped get him into running,” Rees says. “I’m so proud of him now. He’s getting really fast, which is fun to watch.”
A stronger immune system, it seems, does not necessitate stronger sibling rivalry.