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Thursday, 15 November, 2001, 04:28 GMT
Firstborn 'more prone to allergies'
Firstborn children have higher levels of a key immune protein
Firstborn children "have higher levels of a key immune protein"
A firstborn child is more likely to develop allergies than his or her younger siblings, a scientist has suggested.

Dr William Karmaus, of Michigan State University in East Lansing, said the pattern may be partly due to hormonal changes women go through over successive pregnancies.

Research by Dr Karmaus and his team, featured in the New Scientist magazine, indicates firstborn babies have higher levels of a key immune protein - immunoglobulin E (IgE) - which is associated with allergies.

Earlier studies showed that children born with high levels of IgE in their umbilical cord blood have an increased risk of developing allergies.

A skin test for allergies
A skin test for allergies
Dr Karmaus said: "The question was, can this be related to birth order?"

But his study indicated firstborns did have higher levels of IgE.

Dr Karmaus speculates this could be linked to levels of a chemical in the placenta which is also associated with the number of children a woman has had.

Family patterns

The team looked at the IgE levels of almost 1,000 children born on the Isle of Wight between 1989 and 1990.

They found that16.5% of firstborns had IgE levels of higher than 0.5 kilo units per litre, while only 12.8% of children with one older sibling had such high levels.

Only 8% of children with two or more older siblings had these high levels.

The researchers suggest this may mean that instead of benefiting from being exposed to their older siblings' germs as they grow up, the advantage may actually begin in the womb.

They suggest that attempts to develop vaccines mimicking a dirty environment might not therefore be the complete answer.

The study also found that while 37.5% of the firstborns with high IgE levels at birth were sensitive to a skin prick allergy test, only 23.5% of the children who had similarly high levels of IgE in their cord blood but more than one sibling had an allergic reaction.

Dr Karmaus suggested the reason for the decreasing levels of IgE in successive pregnancies could be linked to the pregnancy itself.

He said previous studies had shown high levels of IgE in umbilical cord blood were linked to raised levels of a chemical called organochlorine in the placenta, which disrupts hormone production.

Levels of this chemical in breast milk have also been found to be lower after later births.

Germs and dirt

It was in the 1980s that it was first spotted that he bigger a family, the less likely its youngest members were to have allergies such as asthma, hay fever and eczema.

This pattern was explained by the so-called hygiene hypothesis, where being in too clean an environment means children's immune systems work less well, and they are more likely to develop allergies.

Scientists backing the hypothesis suggest that having older siblings bringing germs and dirt into the home means a child less likely to get allergies.

But Anne Wright, an asthma epidemiologist at the University of Arizona in Tucson, said: "The relationship between birth order and cord blood IgE cannot possibly be attributed to infections by being around other children."

However she said the findings on skin prick allergy tests suggested Karmaus and his team had discovered an additional reason for the sibling effect, rather than the hygiene hypothesis being wrong.

She was backed by Professor John Warner, director of the Allergy Information Sciences Division at the University of Southampton School of Medicine.

He told BBC News Online the explanation for the pattern was more likely to be linked to changes to a mother's exposure to germs from her older children while pregnant rather than hormonal changes.

"The findings are valid. But the interpretation is wrong, and it is inappropriate to dismiss the hygiene hypothesis," he said.

The study is also published in the American Journal of Epidemiology.

See also:

27 Oct 01 | Health
Asthma vaccine hope
27 May 01 | Health
Keeping pets 'prevents allergies'
11 Feb 00 | Health
Dirt could be good for you
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