Despite the strong and consistent association between immediate hypersensitivity, allergy, asthma and exposure, there is still controversy about the role inhaled allergen plays and about the timing of events related to sensitization. However, IgE antibodies are essential for the asthmatic response and the symptoms are much more closely associated with IgE antibodies to perennial than to seasonal allergens. Although there have been many changes in our environment, none of these alone explains the consistency or the scale of the rise in asthma over the 40 years between 1960 and 2000.