After recording birth orders, the researchers tested each newborns allergic status by examining indicators present in umbilical cord blood. As well, they conducted standard skin prick allergy tests at both age 4 and age 10.
The authors found that among firstborn children, the presence of a particular gene strain -- known as the IL-13 gene variant -- was associated with a higher risk for having an "allergic response." This link continued to persist a decade later.
By contrast, among second or later-born children no such association between IL-13 and higher risk was found. In fact, the role of IL-13 seemed to "switch over" to that of a risk protector.
"The fetus is, in effect, a foreign body," noted Karmaus. "And a foreign body can be exposed to a lot of immune arousal or not, depending. So we think that something during pregnancy -- probably the immune system of the mother -- stimulates the IL-13 gene to act differently, depending on birth order. We haven't shown how this works yet, but that's the idea."
Karmaus suggested that the finding could theoretically lead to the crafting of interventions -- perhaps therapeutic, perhaps simply lifestyle changes -- which could reduce the allergic response risk for firstborns.
Meanwhile, Ly and her colleagues explored similar risks associated with Caesarean sections by analyzing the cellular immune regulatory activity present (in the form of so-called treg cells) in the umbilical cord blood of 50 babies born by Caesarean and 68 babies delivered vaginally. All the babies had a least one parent with allergies and/or asthma.
The authors found that among C-section babies, treg cells were more likely to fail to operate properly, raising the risk for the early onset of immune system disruption. This, in turn, may increase the lik
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