Background: An association of allergic sensitization with small families and low birth order has been described and attributed to a protective effect of early infection. The influence of like-sex and unlike-sex siblings has not been investigated, although the severity of viral infections may be greater if acquired from unlike-sex siblings.
Objective: To investigate the association of self-reported inhalant allergy with family composition.
Methods: Reports of allergy to grass, dust or cats by 11042 pregnant women recruited to a longitudinal study of pregnancy and childhood in Avon, UK, were analysed in relation to respondent's age, maternal age and sibship composition (older and younger brothers and sisters) by multiple logistic regression.
Results: The prevalence of self-reported inhalant allergy decreased with increasing numbers of brothers (test for trend: P < 0.0001), but was unrelated to the number of sisters. The unadjusted prevalences for subjects with none, one, two and three or more brothers were 26%, 23%, 20% and 17%, respectively. The corresponding prevalences for numbers of sisters were 23%, 24%, 22% and 23%. After adjustment for total sibship size, offspring of older mothers were more likely to report allergy (test for trend: P < 0.001), but there was no association with position in the sibship.
Conclusion: Although it is not possible to determine whether brothers specifically, or unlike-sex siblings in general, are inversely associated with inhalant allergy, these findings are consistent with the hypothesis that patterns of sibling interaction within young families influence the risk of future aeroallergen sensitization.