The Hygiene Hypothesis
Reported by Mary Allen
At the CSACI Annual Scientific Meeting held in Ottawa in October 2004, there was an interesting debate on the Hygiene Hypothesis by two experts in the field of allergy. These are some of the main arguments.
Pro: "Hygiene Drives the Process" (Dr. F. Martinez, Tucson, Arizona)
The traditional thinking about allergy was that it develops from a complex interaction of genetics and exposure. This would mean that a high exposure to allergen in early life would lead to more allergy and asthma, which may not always occur. Recent research suggests that the basis for allergy may be more complex. Genetics alone cannot explain recent increases in asthma. Early asthma may depend not just on genetics and exposure to allergen but by a broader range of environmental factors that affect the development of the immune system.
The Hygiene Hypothesis is one theory that may help to explain the relationship between the environment itself and the development of allergy. It suggests that the immune system is affected by exposure to endotoxin in the environment. Immune tolerance to potential allergens could result when exposure to certain microbial derivatives occurs at certain stages during the maturation of the immune system. Less exposure to endotoxin in dirt would result in more allergies. In other words, there is a preventative benefit in being in a not-too-clean environment, at least in certain people. There is some evidence that children on farms have fewer allergies and that some with pets have fewer allergies. However, timing and genes would still play a large role.
Con: "Hygiene is Just an Interested Partner" (Dr. A. Custovic, Manchester, England)
Hygiene is one of many theories that attempt to explain the increased incidence of allergy and asthma (which are not interchangeable). The important question is whether we have a more toxic environment or a more susceptible population. It is certain that allergen exposure alone does not explain the increased incidence, but the hygiene theory is not sufficient as an explanation. It may not be the main factor behind the protective effect of living on a dairy farm and it does not explain increased incidence of asthma in poor inner city neighbourhoods.
With respect to pet allergen exposure: is it the pet allergen that is protective for some individuals or is it the endotoxin that pets inevitably carry?
We need to move away from blanket advice directed to the whole population and tailor advice to particular individuals. And, asthma and allergy may well require different preventative measures.
Where does the debate leave us?
Any attempt to predict factors influencing the development of allergy is a complex undertaking. The key point is that microbial products (bacterial and viral) will modify immune responses depending on varying factors, including genetic pre-disposition. However there are many factors at play and there is as yet and may never be a single explanation that will predict the reason for allergy in all situations. More research will, we hope, sort out what is best for any individual living in any specific environment. In the meantime we will follow the research with interest and continue to encourage individuals with allergy to seek professional care and to adopt the strategies and treatments that give them the most comfort.
from Allergy & Asthma News, Issue 2 2005