Jaarboek no. 87. 2008/2009 - Koninklijke Maatschappij voor ...

Introduction
IMMUNOLOGIE VAN DE HYGIËNE-HYPOTHESE
Immunology of the hygiene hypothesis
door
Mw. prof. dr. M. Yazdanbakhsh
Afdeling Parasitologie, Leids Universitair Medisch Centrum, Leiden
Many studies have documented that the prevalence of sensitisation (IgE and skin prick test
positivity) to allergens has been increasing, not only in industrialised developed countries
but also in less affluent developing countries (ISAAC III). However, there is considerable
geographic variation in the prevalence of allergic disorders and although genetic factors
may play a role in the development of allergic disorders, they are unlikely to explain the
strong urban/rural differences that is seen in the prevalence of allergies. So far, epidemiological
studies examining the geographical variation in the prevalence of allergic disorders
have suggested that rural lifestyle confers protection, whereas urbanisation is an important
risk factor for diseases such as asthma and atopic dermatitis. In rural Ethiopia, South
Africa, Kenya and Gambia but also in rural traditional farmers residing in Europe (for
example in Germany or in Eastern Europe), there is little allergy compared to major urban
cities. It has been proposed that understanding the cause of the change in prevalence of
allergies would help us understand the risk factors at the environmental and mechanistic
level that are responsible for the so called ‘allergic march’ worldwide. This knowledge
can then be exploited to stop the allergy epidemics worldwide. One of the environmental
factors, namely infections, is thought to play a role in this regard: although some respiratory
infections are known to exacerbate asthma, there is a list of other infections that are
thought to play a protective role.
Exposure to micro-organisms and parasites: risk or protective factor
During the course of evolution man has constantly been exposed to a wide variety of micro-organisms
and parasites, ranging from harmless to life-threatening ones. Our immune
system has evolved to counteract dangerous infections but to tolerate or even to depend
on harmless ones. For example, the gut microflora, is essential for the normal development
of the immune system as shown in studies of animals raised under germ free conditions.
Studies of the interaction of the immune system with molecular patterns present
on bacteria in the gut have shown, for example, bacterial polysaccharide to be essential
for the development of a fully functional immune system. In line with this, there has been
much interest in, and controversy over, the relationship between infections and allergies.
The Hygiene Hypothesis
The hygiene hypothesis was initially proposed by David Strachan when he observed that
in a British Cohort study allergic rhinitis was more prevalent among first-borns and less
Natuurkundige Voordrachten Nieuwe reeks 87. Lezing gehouden voor de Koninklijke
Maatschappij voor Natuurkunde ‘Diligentia’ te ’s-Gravenhage op 23 maart 2009.
Immunologie van de hygiëne-hypothese