Immunocompetence and allergy.

Developed nations are experiencing a marked increase in prevalence of the familial allergic diseases including asthma, allergic rhinitis, atopic dermatitis, and allergic gastroenteropathy, which are often called atopic diseases. No satisfactory explanation for this epidemic is known, but it has been proposed that some facets of modern life tend to bias immune responses away from the Th1 cellular immune responses that protect against many infections and toward Th2 responses that favor atopy. There are 2 hypotheses to explain why this epidemic is occurring now. Hypothesis 1 suggests that nutritional patterns have changed or that we are exposed to environmental toxicants that were not previously present. Hypothesis 2 holds that some aspects of modern lifestyles in affluent nations have minimized exposure to infectious agents or to their by-products, such as endotoxin. This feature of contemporary lifestyle, it is suggested, has favored the development of Th2 immune responses to environmental allergens and the development of the attendant atopic diseases. This latter theory has been designated the "hygiene hypothesis." Although there is evidence both for and against both hypotheses, evidence for hypothesis 2 is stronger and more convincing.