Early exposure to allergen: is this the cat's meow, or are we barking up the wrong tree?

Several recent studies have suggested that exposure to cat and dog allergen in infancy is protective against the subsequent risk of allergic sensitization and asthma. The methodologic problems to be overcome in clinical research addressing these hypotheses are complex. Appreciation of these studies requires an assessment of the design and adequacy of variables measuring exposures, outcomes, and confounders. It includes understanding the role of effect modification. This article discusses some of the epidemiologic issues in interpreting these studies. Review of relevant epidemiology demonstrates that much research remains to be performed before these interesting hypotheses are proved or disproved.