[The role of the environment in asthma: hypotheses and contradictions].
Three hypotheses are described to explain the relation between allergens and environmental co-factors and the onset of atopy: the hygiene hypothesis, the allergenic hypothesis, and the high exposure tolerance inducing a Th2 derived response with blocking IgG4 synthesis. None of these hypotheses have been confirmed. It seems thus difficult to give recommendations for primary prevention of allergic diseases until results of prospective studies allow to consider a more precise behaviour. In contrast, subjects sensitized and exposed to allergens present an increased risk to develop asthma or non specific bronchial hyperreactivity. Therefore, secondary prevention appears as an essential method for treatment of allergic disease, with clinical benefits on symptoms which have recently been demonstrated in a clinical study.