Hygiene hypothesis

Hygiene Hypothesis is a theory suggesting that a lack of early childhood exposure to infectious agents, symbiotic microorganisms (such as the gut flora or probiotics), and parasites increases susceptibility to allergic diseases by suppressing the natural development of the immune system. In particular, the lack of exposure is thought to lead to defects in the establishment of immune tolerance.

Overview

The Hygiene Hypothesis was first formulated in 1989 by David P. Strachan who observed that hay fever and eczema, both allergic diseases, were less common in children from larger families, which were presumably exposed to more infectious agents through their siblings, than in children from families with only one child. Strachan suggested that the lower rates of hay fever and eczema among larger families could be explained by the "hygiene hypothesis".

Development and Variations

The Hygiene Hypothesis has been extensively investigated by immunologists and epidemiologists and has become an important theoretical framework for the study of allergic disorders. It is used to explain the increase in allergic diseases that have been seen since industrialization, and the higher incidence of allergic diseases in more developed countries. The Hygiene Hypothesis has now expanded to include exposure to symbiotic bacteria and parasites as important modulators of immune system development, along with infectious agents.

Evidence

Epidemiological evidence supporting the Hygiene Hypothesis includes the lower incidence of asthma and atopic disease in children from larger families, in children who were breastfed, in children living on farms, and in children who were exposed to pets or livestock in early childhood.

Criticism

Critics of the Hygiene Hypothesis suggest that it is too simplistic as it does not account for the complex nature of the immune response and the factors that influence it. They also argue that the hypothesis does not explain why allergies are less common in developing countries, where hygiene levels are typically lower than in developed countries.

See Also

• Allergy
• Asthma
• Eczema
• Hay Fever
• Immune System
• Infectious Disease

References

Hygiene hypothesis Resources
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