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Can early infection explain the sibling effect in adult atopy?

¹ Depts of Occupational and Environmental Medicine and ² Microbiology, Imperial College School of Medicine (NHLI), London, UK.

CORRESPONDENCE: P. Cullinan, Dept of Occupational and Environmental Medicine, Imperial College School of Medicine (NHLI), Manresa Road, London, SW3 6LR, UK. Fax: 44 2073518336. E-mail: p.cullinan@ic.ac.uk

Keywords: atopy, early infection, family size, hygiene

Received: May 10, 2003
Accepted July 22, 2003

The study was supported by the Colt Foundation.

Atopy is strongly and inversely related to family size, a pattern which is plausibly assumed to reflect a protective effect of early infection. The current study tested this hypothesis by case-referent analysis of an adult cohort in the UK.

The study established that atopy, defined by prick tests to common aeroallergens, was less common among those from larger families after adjustment for potentially confounding factors. In particular, a higher number of brothers appeared to offer protection. The current authors attempted to explain this distribution by examining contemporary family-doctor records of early childhood infections; and by a number of other indirect indices of early-life "hygiene".

The sibling effect was unexplained by evidence of infection with either hepatitis A or Helicobacter pylori, or by counts of infections or antibiotic prescriptions in early life. There was a significant and independent negative association between the number of gastrointestinal infections before the age of 5 yrs and the odds of atopy. Dog ownership and home moving in early life also displayed potentially protective associations.

Although the current study replicates the finding that atopy is inversely associated with family size this could not be explained by documentary or serological evidence of early infection. The findings support the suggestion that the "sibling effect" in atopy may not simply reflect protection by early infection.

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