asthma and early exposure to allergens

lots of interesting articles, several sent out, over the past year on the microbiome and on allergen exposures and subsequent allergies. recent one of the Urban Environment and Childhood Asthma study (URECA), done in several urban areas (baltimore, boston, new york, st louis), a study of 560 kids at high risk of developing asthma and followed since birth for 3 years with environmental assessments including allergen exposure.  also, a second study looked at 
a nested case-controlled study of 104 kids assessing the bacterial content of their house dust exposure -- see asthma early exposure allergens jallergclinimmun 2014 in dropbox, ordoi.org/10.1016/j.jaci.2014.04.018. results:

    --cumulative allergen exposure over first 3 years of life associated with allergic sensitization and related to recurrent wheezing
    --BUT, first-year exposure to cockroach, mouse and cat allergens was negatively associated with recurrent wheezing (ORs of 0.60, 0.65, and 0.75 respectively and all with p<=0.01). also, the probability of asthma decreased additively with increasing numbers of these allergen exposures. however, house dust mite and dog allergens had no association with asthma at 3 years old. 
    --differences in house dust bacterial content in the first year was associated with differences in atopic wheezing, esp with reduced exposure to Firmicutes and Bacteriodetes being associated with increased atopy. conversely, exposure to high levels of these allergens was associated with less asthma. the combination of high exposure to these bacteria in combination with high allergen exposure was additively protective of developing asthma. Presumably exposure to these bacteria protects against atopy since they come from bacterial families which are human colonizers and important producers of immunomodulatory metabolites, with presumption that early-life exposure to house dust containing these bacteria could innoculate the GI and/or respiratory microbiomes.

i think we are getting closer to the full story of the relationship between allergen exposure and allergies in kids. it has been hard to reconcile the many studies finding increased asthma in inner city environments (and the above study found that an annual family income <$15K was associated with increased asthma) with studies showing decreased asthma in those with high early allergen exposure. this study suggests that there may be important differences depending on the specifics of exposures, and that these exposures specifically need to be within the first year of life (presumably in the setting of an immature immune system). other microbiome studies suggest that the specific foods eaten can affect the specific bacterial content of the gut (and the lung, in some cases) and effect clinical issues such as airway resistance. of course, there are other precipitants for allergic asthma as well, including stress and exposure to other indoor pollutants.

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