Background: Having a low number of siblings and a low birth order has been reported to be a relevant risk factor for development of atopic diseases and skin sensitization to common inhalants. Although the inverse association of atopy with sibship size has been confirmed repeatedly, the association with birth order has provided conflicting results. This possibly is due to the relatively small size of the population sample examined.
Objective: The objective of this study was to investigate the relation between sibship size, birth order, and atopy in a very large population sample, highly homogeneous for age and sex.
Methods: This was a retrospective survey of 11,371 Italian young men, 18 to 24 years old, all candidates for enrollment in the Italian Air Force. Demographic data had been collected by a standard questionnaire. Specific IgE for locally relevant airborne allergens had been tested by a multi-RAST assay (CAP-Phadiatop).
Results: The prevalence of atopy (defined as a high level of specific IgE against inhalants [cut-point >1.2 log RU]) was inversely related to the total number of siblings (25% in those with no siblings and 9% in those with five or more siblings), with a mean of a 3% decrease in prevalence for each added sibling. This relation persisted after adjustment for relevant variables such as father's education and rural and southern residence. An independent association between birth order and atopy was also observed because the decrease in atopy prevalence with increasing numbers of older siblings was significantly steeper than that found with the number of younger siblings (chi2 = 179, df = 1, p < 0.0001).
Conclusions: In a very large and homogeneous population sample of a Mediterranean country, not only sibship size but also birth order was significantly associated with atopy. This observation further highlights the role of family structure in the development of atopy and supports the hypothesis that cross-infections acquired early in infancy or in later childhood might prevent development of atopy later in life.