Total and specific IgE: Total serum IgE and IgE to common allergens (house dust mites, grass pollen and cat dander) will be measured by enzyme immunoassay (Unicap System, Pharmacia UK Ltd, Milton Keynes, UK). Total IgE > 120 IU/L and specific IgE >0.35 IU/L are considered positive. Atopy will be defined as the presence of positive specific IgE to common allergens.