In general, evidence is weak or nonexistent supporting connections between pet dander or fungi and asthma development. Clearly, pet dander and fungi exacerbate asthma in individuals sensitized to a particular allergen, but evidence is lacking that pet dander contributes to asthma development, and only recently have preliminary studies started to make a connection towards fungi exposure and asthmagenesis (National Academy of Sciences 2000).
Perhaps the best evidence linking in utero exposures to allergen sensitization in infancy comes from a report showing that exposure to birch pollen during the last
2 to 3 months of pregnancy is associated with much less birch pollen reactivity of the offspring than occurs if the exposure was between 3 and 6 months' gestation (Van Duren-Schmidt et al. 1997). As the authors of that study pointed out, of particular relevance is that IgG transfer across the placenta is maximal during the latter few months of pregnancy, so maternal/fetal interactions may be playing a major role in this case. Moreover, other studies have shown that children are at increased risk for allergy to seasonal allergens if born shortly before the relevant pollen season, when protective maternal (and presumably fetal) IgG antibodies would be at their lowest levels (Jenmalm and Bjorksten 2000). It will be particularly interesting to learn from future studies if there is a concomitant increase in risk for the development of asthma, since many of the same allergens are involved.
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