New research finds that different conditions in the womb during pregnancy mean that first-born children are more likely to develop asthma and other allergies than their siblings.

In the first study of its kind a team of a team of researchers from the University of South Carolina studied 1200 newborn babies and their mothers from the Isle of Wight (United Kingdom). The scientists took samples of blood from the umbilical cord to look for indications of how well the immune system was functioning and for specific immune chemicals.

They measured levels of Immunoglobulin E (IgE), a chemical which is instrumental in bringing about the classical allergic response. Its presence triggers the release of histamine from certain cells in the body. Histamine being the chemical responsible for the common symptoms of allergies.

A high level of IgE in the cord-blood would suggest a child has a predisposition to developing allergies as they grow up.

The researchers followed up the IgE blood testing with skin prick testing to determine the presence of allergies first when the children had reached age 4 and again at age 10. They could then look for an association between IgE levels when they were born and the chances of them developing allergies as they got older.

One of the aims of the study was to determine whether the effect of certain genes involved in the development of asthma and allergy differs between first-borns and their brothers and/or sisters.

Once the data had been collected and analysed it was found that among first-borns, those with a specific genetic variation were 88% more likely to have increased cord-blood IgE. This was associated with them being 67% more likely to have a positive skin prick test at age 4 and 50% more likely to test positive at age 10 (it is common for children to outgrow some of their allergies).

The researchers did not see the same associations when they looked at data for second and third born children. It seems that the genes involved in the development of asthma and allergy work differently in first-borns.

The findings were presented by the research team at the American Thoracic Society’s 2008 International Conference in Toronto yesterday.

Dr. Wilfried Karmaus said: “We were not surprised that birth order had an effect on the development of the immune system, but were surprised that this interaction persisted at least through age ten."

“Our findings add to the evidence that allergic reactions are programmed during pregnancy and then effect the disease in later life."

The researchers suggested that their findings might help explain the increasing prevalence of atopic disease (asthma, allergies) in developed nations. Birthrates in these countries is declining which means a larger proportion of children are first-borns.

The study also suggests it may be possible to reduce the chances of a first-born child developing asthma and allergies by altering the conditions in the womb.

Dr. Karmaus concluded: “If we could produce pregnancy conditions for first-born children that are comparable to later-born children, we may be able to prevent 20 to 30 percent of all cases of asthma and allergy.”

“Better understanding the mechanism involved in prenatal priming [of immune development] may offer new and exciting avenues of intervention.”

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