Risk factors in allergy/asthma.

The incidence of allergic rhinitis and asthma is increasing. Nobody knows why with certainty, but there are many theories including better diagnosis, urban living, higher exposure to dust mites, atmospheric pollution, nutrition, lifestyle changes, maternal smoking, diesel fumes, geography, the "hygiene hypothesis," and several others. It has been known that atopic disease runs in families and a family history is the strongest risk factor for the development of allergies/asthma. Molecular studies also have identified some specific atopic entities that are determined genetically (eosinophils, interleukin-5, etc.). Environmental issues and lifestyle changes are becoming increasingly more important as significant risk factors but the evidence can be confusing, controversial, and even contradictory. There is overwhelming evidence that sensitization to indoor allergens is a major risk factor for the development of clinical atopic disease in genetically susceptible individuals. Not everyone agrees and there are newer data to suggest that early exposure to endotoxin and/or living on a farm and even early exposure to cats and dogs protects against sensitization by driving the immune system to a TH1 lymphocyte response. To this point, the new data are impressive if not compelling. The "hygiene hypothesis" suggests that lack of exposure to a childhood infection, endotoxin, and bacterial products is an important determinant regarding development of atopic disease. It is clear that a family history (genetics) is the strongest risk factor for the development of clinic atopic disease but it is also clear that environmental issues play a significant role and that there is a lot we still do not know.