Thursday 03 December 2009, 17:35

Allergy incidence: the hygiene hypothesis

A new hypothesis explaining the recent rapid rise in allergic disorders

Allergy incidence: the hygiene hypothesis

The incidence and prevalence of allergic disorders have increased dramatically over the last 30-40 years, apparently without any specific explanation.


Genetic predisposition is a contributing factor in allergy susceptibility; however the increase in incidence of allergic disorders has occurred in too short a time to result from genetic changes in the population. That’s why research has focused on environmental and/or lifestyle factors to establish a plausible cause of this sudden and dramatic rise.


One of the hypothesis formulated to explain the recent rapid rise in allergy incidence is the so-called “hygiene hypothesis”. In simple words, the hygiene hypothesis states that the major causative factor in the increasing incidence of allergy is the reduced microbial exposure of people (in particular children) due to increased sanitation and cleaner lifestyles.


The major strength of hygiene hypothesis is its biological plausibility. At birth, the immune system of neonates appears to rely primarily on the Th-2 system, which is responsible for the production of antibodies against invading microbes, but which also drives allergic reactions in children and adults. In nonallergic individuals, Th-2 responses gradually decrease in the first two years of life and are substituted by Th-1 responses, which are particularly effective against intracellular infections. However, in order to develop, Th-1 system needs to be stimulated by environmental stimuli (such as environmental or endogenous bacteria).


Therefore any practice intended to reduce the environmental bacterial concentrations (such as sanitation, water treatment, excessive cleaning) or accidentally upsetting the normal endogenous flora (such as prolonged antibiotic therapy), may have a negative impact on the normal development of Th-1 system and lead to the persistence and the increase of Th-2 responses which, as already said, are involved in allergic reactions.


The hygiene hypothesis has raised considerable debate, especially among infectious disease specialists, concerned about its implications for hygiene (in the home as well as in the outdoors) and for infectious disease prevention. Therefore research is still moving on both taking into account other factors that could explain the current allergy trends and defining more accurately the nature of the possible link between microbial exposure and allergy.


By Chiara De Carli 

Category: Allergy

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