Genetics and pollution are the major causes of allergies, but this does not explain the regular increase in the rate of allergies found in western countries.
For a long time, excess hygiene was the culprit, however we now know that several factors are involved in triggering the allergic reaction, excess hygiene included, and all these factors lead to a change in our microbiota.
You will find below a set of flashcards about allergies. (in French only)