Atopy is the tendency to produce an exaggerated immunoglobulin E (IgE) immune response to otherwise harmless substances in the environment.

[2] Atopy may have a hereditary component, although contact with the allergen or irritant must occur before the hypersensitivity reaction can develop (characteristically after re-exposure).

[13] In an allergic reaction, initial exposure to an otherwise harmless exogenous substance (known as an allergen) triggers the production of specific IgE antibodies by activated B cells.

[13] However, upon re-exposure, the allergen binds to membrane-bound IgE which activates the mast cells, releasing a variety of mediators.

[4] This hypothesis proposes that excess 'cleanliness' in an infant's or child's environment can lead to a decline in the number of infectious stimuli that are necessary for the proper development of the immune system.

A Swedish research study titled "Atopy In Children Of Families With An Anthroposophic Lifestyle" comparing the rate of bronchial asthma, allergies, dermatitis, and other atopic diseases among Steiner school pupils and pupils in public schools originally appeared in the May 1, 1999, edition of the British medical journal The Lancet.

The researchers investigated a variety of factors in the lives of the Steiner school pupils that might have contributed to this lower rate of atopy, which included breastfeeding, reduced immunization, avoidance of antibiotics and medications that reduce fevers, consumption of bio-dynamic and organic foods, and other physical aspects of the children's lives.

[26] A hallmark indicator of atopic eczema is a positive wheal and flare reaction to a skin test of S. aureus antigens.

[27][28] In adults, the prevalence of IgE sensitization to allergens from house dust mite and cat, but not grass, seems to decrease over time as people age.