OBJECTIVE: To determine whether increased numbers of siblings and infection in early life protect against allergic sensitisation. DESIGN: Historical cohort study. SETTING: Sheffield, UK. SUBJECTS: 11,765 children aged 11-16 years for whom a history of neonatal infectious illness had been recorded systematically at 1 month of age. METHODS: A history of hay fever and family structure was obtained by postal questionnaire; neonatal illness history was ascertained from health visitor records; 723 children underwent skin prick testing with mixed grass pollen extract. RESULTS: The prevalence of hay fever was reduced (p < 0.0001) among children of younger mothers, and those from larger families. The number of older siblings exerted a stronger independent effect than the number of younger siblings (p < 0.001). Infants breast fed exclusively during the first month were at higher risk (p < 0.05) of subsequent hay fever, independent of demographic factors. Adolescents at high risk of hay fever by virtue of their family structure were more likely to be sensitised to grass pollen (p < 0.002). No significant relations emerged between hay fever and infection in the first month of life, even among children born in June. CONCLUSIONS: The association of hay fever with family structure is not due to reporting bias and reflects an environmental influence on allergic sensitisation. The effects of sibship size, birth order, and infant feeding are consistent with a protective influence of postnatal infection. The first month of life and the first postnatal exposure to allergen are not the critical periods during which this protective effect is determined.
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