TORONTO - 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 specific immune-response
substance in their umbilical cord blood than did second-born children. The
substance is an antibody called IgE.
"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 more than 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," he speculated, creating a higher risk of the child's
immune system reacting to allergens.
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."
Commenting on the research, allergist Dr. Susan Waserman of McMaster University
called the research interesting but by no means definitive.
The findings suggest that some process could be occurring in the uterus that
affects first-borns and may represent an intervention point for preventing
allergy, she said from Hamilton.
"But at this point it's just the subject of interest and future investigation."
