Epidemiology of the allergic response

Asthma and related common allergic diseases are significant worldwide health problems which cause considerable morbidity and appreciable mortality and make severe demands on the time and costs of medical and hospital care. The more readily available epidemiological data relate to asthma, which has a wide prevalence, ranging from a general figure of 4-8% for developed countries to less than 1% in some European, African, Indian and Melanesian countries. Earlier reports that asthma is uncommon in the tropics, particularly in rural areas, appear from later studies to be an over-generalization; the highest prevalence of asthma in the world has been reported in the Western Caroline Islands, the Maldives and Tristan da Cunha. Atopic diseases are associated with excess IgE production, which is genetically controlled, and it is likely that additional genes control allergic shock organs. However, the development of allergic disease is balanced between genetic and environmental factors, principally exposure to aggressive allergens, which is particularly relevant during the first six months of life, amplified by maternal smoking both pre- and postnatally, and IgA deficiency in breast milk. Other factors such as viral infections and atmospheric pollution play important but lesser roles. A unique feature of asthma in village communities in developing countries, in contrast to developed, is that it is much less frequent in children than in adults. Although this may be relevant to understanding the regulation of allergic disease in general the mechanism(s) remain to be determined.