The causes of the increasing prevalence of allergy: is atopy a microbial deprivation disorder?

The atopic diseases, i.e., primarily, bronchial asthma, atopic dermatitis, and allergic rhinoconjunctivitis, were rare a few decades ago, but constitute today an increasingly severe public health problem.The increase in the prevalence of the allergic diseases is mainly seen in children (1).Recently, the ``European Allergy White Paper'' described the worrying increase in allergic diseases in Europe (2).Formerly, allergic diseases such as hay fever were considered an exclusively upper-class disorder.In 1828, John Bostock described the hay fever symptoms in himself and in 27 other persons that he had found after meticulous scrutiny of all the clinics of London.In 1862, Philip Phoebus published a cogent monograph on hay fever based on data from 300 cases from the medical journals.He described the geographic as well as hereditary and social factors affecting his patients.In a book published in 1873 (3), Charles Blackley, himself a hay fever sufferer, provided clear evidence that pollen grains are the causative agents of the disease.He also claimed that catarrhus aestivus, also called hay fever or hay asthma, was more common among the educated than among the illiterate.He noted a curious circumstance regarding hay fever, namely, that the people who were most likely to be subjected to the action of pollen belonged to a social class which furnished the fewest cases of the disorder, i.e., agricultural laborers.Remarkably, Blackley not only made the interesting observation of the association of hay fever and lifestyle but also predicted that ``as civilisation and education advance, the disorder will become more common than it is at the present time''.This is indeed what has happened.The increase in the prevalence of the allergic diseases, especially in those born after 1960, is almost explosive, and there are now epidemics of allergic diseases in many countries.The prevalence of asthma in children and young adults has tripled and quadrupled in many industrialized countries during the last two decades (1).It is often pointed out that the increase of the allergic diseases is associated with ``westernization'' of lifestyle.It is thus obvious that the atopic diseases correlate with socioeconomic levels.Generally, it seems clear that atopic diseases, especially hay fever, are more common among the wealthy than among the poor, in urban than in rural areas, and in the West than in the East (4±6).Regarding the potential causes of the increase in the allergic diseases, it is important to remember that the induction of allergy, i.e., allergic sensitization, and the precipitation of allergic symptoms are two completely different phenomena.Therefore, it is theoretically possible that the same factor might protect against allergic sensitization, but also, if such sensitization has already occurred, induce allergic symptoms.An analysis of the possible causes of the increased prevalence of allergy must be based on the assumption Allergy 2001