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(Canadian Press)  First-born children are known to have a higher risk for developing asthma and allergies than their younger siblings, and researchers suggest part of the reason may lie in the womb.

A study of 1,200 children on the U.K.'s Isle of Wight, followed from birth into childhood, found first-borns had higher levels of a certain immune-response substance in their umbilical cord blood than did second-born children . The substance is IgE, an antibody responsible for allergic reactions.

"This was important because there are two different schools of thought," lead author Dr. Wilfried Karmaus, an epidemiologist of the University of South Carolina, said in an interview. "One is considering the hygiene of the delivery of the birth is important for the immune (system) development and the other thought is that the prenatal exposure during pregnancy is important."

"And we could show that the immune markers (the IgE antibody) in first-born are already higher compared to second-born," he said. "I think it's not the hygiene, but the prenatal programming that plays a role."

(The hygiene hypothesis suggests that first-born children are not exposed to as many germs as their younger siblings are, so their immune systems overreact to common substances like grass, pollen and dust as if they were infectious agents.)

The study, presented at the American Thoracic Society's annual international conference in Toronto, also showed that the elevated risk for asthma and allergies persisted in eldest children throughout childhood.

At age four and 10, those with a higher amount of the antibody in their cord blood were more likely to have a positive skin-prick test for allergies and asthma compared with a younger sister or brother, Karmaus said Tuesday.

First-borns had about double the risk of reacting to skin-prick allergy testing than did their younger siblings, the study found.

Karmaus said the amount of IgE is linked to a specific gene. In a first pregnancy, this gene appears to be more active, leading to a greater expression, or output, of the antibody.

"It's unclear what is the driving force behind it or what is the potential of risk for increased cord-serum IgE," said Karmaus, noting that women giving birth to their first child also have a higher blood level of the antibody than they do when having a second child.

"We assume that the contact between mother and child leads to a more immunological upset or arousal in the first pregnancy, so that something in the gene becomes expressed," creating a higher risk of the child's immune system reacting to allergens, he speculated.

Commenting on the research, allergist Dr. Susan Waserman of McMaster University called the research interesting but by no means definitive.

"This whole concept about first-borns having higher prevalence rates of allergy and asthma is not new," she said from Hamilton. "The explanation we've had for this is that older children have less exposure to infections," and hence a greater tendency to allergy, "since the immune system becomes disregulated when there's not enough infection."

Waserman said it's difficult to know how to position the new research.

"You're already finding evidence of allergy," she said, referring to elevated antibody levels in first-born children's cord blood.

"So there's some gene environment interaction that's taking place as early as in-utero. I just don't know how much more we can say about it at this stage of the game because we don't know what the interaction is."

Karmaus estimated that 20 to 30 per cent of all cases of asthma and allergy could be prevented if pregnancy conditions for first-born children could be altered somehow, although how that could be done is unknown.

"We don't know (how) yet, but the idea is to produce a condition during the first pregnancy which is comparable to a second pregnancy."

Waserman said the findings suggest that some process could be occurring in the uterus that affects first-borns and agreed that - one day - it might represent an opportunity for preventing allergy.

"But at this point," she said, "it's just the subject of interest and future investigation."

Source: Canadian Press
Author: Canadian Press