Reacción a "Reaction: Research links exposure to pets from pregnancy to early childhood with fewer food allergies in babies"

This is a large sample size study showing an association between exposure to companion animals during childhood and the risk of food allergy incidence up to the age of three years. The results of this work support previous research indicating that certain environmental exposures, such as children growing up with an older sibling or in an agricultural environment during the first years of life, are associated with greater bacterial diversity and a lower risk of allergic sensitisation. This is known as the hygiene hypothesis. 

However, this is a retrospective study, i.e. existing data have been used to compare between two groups, allergic and non-allergic. The authors themselves recognise the limitations of this type of study when it comes to establishing a cause-effect relationship, as there are other uncontrolled factors that could influence the results, such as genetic predisposition to suffer from some type of allergic condition or certain environmental factors. Furthermore, in this study they observed that the association between exposure to pets and food allergies differs according to the species of animal and the causative food, so it would have been interesting to see the influence of the introduction of allergenic foods in the baby's diet or exposure to them during the first months of life. 

In any case, the study raises an interesting question that needs further investigation and opens the way to design randomised controlled research studies that prospectively assign exposure to pets or to plan basic research to elucidate the immunological mechanism behind this observation.