For the 10-year study, researchers recruited 1,212 newborns and ascertained their birth order.
Samples of cord blood were collected from each child to determine levels of IgE. Skin prick tests were also taken at the ages of four and 10 to determine their sensitivity against a range of allergens.
First-borns had 1.88 times more IgE in their blood than other children and were also more likely to have a positive skin prick test at the ages of four and 10.
Further, genotype testing on 925 of the children revealed that first-born babies had a variation in a single nucleotide of the IL-13 gene, which changed an amino-acid and caused an increased production of IgE.
Lead researcher Dr Wilfried Karmaus, from the department of epidemiology at the University of South Carolina, said the findings showed that allergic reactions are programmed during pregnancy.
‘We believe that birth order affects gene expression during foetal development,’ he said.
‘This finding may partially account for the increasing prevalence of asthma and allergies in children in the last 30 years, primarily seen in the western world, as birth-rates continue to decline.
The findings were presented this week at the annual American Thoracic Society Conference being held in Toronto, Canada.
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