title: The effect of age on the relationship between birth order and immunoglobulin E sensitization.
abstract: BACKGROUND
  An association between birth order and IgE sensitization or allergic diseases has been reported in many studies.
  
  
  OBJECTIVE
  To assess the effect of age on the relationship between reduced IgE sensitization and increased birth order and to test the hypothesis that this would decline with increasing age.
  
  
  METHODS
  As part of a birth cohort study, IgE sensitization to common allergens was determined by skin prick testing at ages 6 and 12 months, 6 and 11 years.
  
  
  RESULTS
  The original cohort numbered 253 individuals of whom 96 (38%) were first born. Compared with individuals with older siblings, first-born individuals had increased IgE sensitization at 6 (odds ratio (OR) 2.4 [95% confidence interval (CI) 1.0, 6.3], P=0.05, n=197) and 12 months of age (OR 6.7 [1.7, 25.0] P=0.002, n=172) and at 6 years of age (OR 2.3 [1.0, 5.6] P=0.05, n=113) but not at 11 years of age (OR 1.2, P>0.4, n=182). When age at onset of IgE sensitization was considered (n=61), 16 had infant onset IgE sensitization (nine were first born), 24 had early childhood onset IgE sensitization (nine first born) and 21 had late childhood onset IgE sensitization (two first born), P=0.0016. Further analysis revealed a similar pattern for children with older brothers (P=0.0097) but not older sisters (P=0.5).
  
  
  CONCLUSIONS
  These findings indicated that having an older brother delays the onset of IgE sensitization but may not prevent IgE sensitization per se. The apparent protective effect of older siblings on allergic diseases reported elsewhere might involve delaying the onset of IgE sensitization.
authors.affiliation: School of Paediatrics and Child Health, University of Western Australia, Perth, Australia. s.w.turner@abdn.ac.uk
authors.forename: S W
authors.initials: SW
authors.lastname: Turner
authors.forename: L J
authors.initials: LJ
authors.lastname: Palmer
authors.forename: N A
authors.initials: NA
authors.lastname: Gibson
authors.forename: P J
authors.initials: PJ
authors.lastname: Rye
authors.forename: J
authors.initials: J
authors.lastname: Goldblatt
authors.forename: L I
authors.initials: LI
authors.lastname: Landau
authors.forename: P N
authors.initials: PN
authors.lastname: Le Souëf
mesh_terms: D017668:Age of Onset; D000375:Aging / Q000276:immunology*; D000485:Allergens / Q000276:immunology; D001722:Birth Order; D002648:Child; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D006967:Hypersensitivity / Q000453:epidemiology / Q000276:immunology*; D007073:Immunoglobulin E / Q000276:immunology*; D007223:Infant; D007231:Infant, Newborn; D008137:Longitudinal Studies; D008297:Male; D015995:Prevalence; D035781:Siblings; D012882:Skin Tests / Q000379:methods
chemical_list: D000485:Allergens; D007073:Immunoglobulin E