A Little Dirt may be Good for Your Immune System
The Hygiene Hypothesis or why Minor Illnesses may Prevent Allergies
Mar 24, 2009
The cells of the immune system protect the body from damage caused by invading pathogens – bacteria, viruses, fungi and parasites. But the immune system doesn’t always get it right; one of its key cells is not getting appropriately turned on, as people worry about cleanliness. The hygiene hypothesis is one explanation for the increasing incidence in the Western World of allergies such as hay fever and autoimmune diseases like rheumatoid arthritis and multiple sclerosis.
Allergies and the Immune System
Allergies arise when the immune system acts against the body; it swings from its traditional friendly role to that of an enemy. An allergic response occurs when the immune system reacts to a substance that is basically harmless or does not cause an immune response in everybody. The immune system goes into overdrive producing antibodies and soluble molecules such as histamines. It is these histamines that are responsible for the sneezing, itchy eyes and throat that characterise many a summer for hay fever sufferers.
Hygiene Hypothesis
The prevalence of allergies is rising in the Western world. The hygiene hypothesis may explain this. The immune system exercises itself eliminating harmful pathogens (viruses or bacteria), but the increasingly clean lives of many people means that they are not stimulating the immune system appropriately.
The hygiene hypothesis proposes that exposure to pathogens early in life reduces the risk of developing allergies by boosting immune system activity. Children on German farms who regularly encountered a range of potential pathogens in the farmyard were found to have a significantly lower incidence of allergies such as hay fever (Debarry, J. Acinetobacter lwoffii and Lactococcus lactis strains isolated from farm cowsheds possess strong allergy-protective properties. J Allergy Clin Immunol (2007), pp 1514-2).
Regulation and Stimulation of the Immune System
Balance in the types of immune cells activated is crucial to understanding these ideas. Scientists have discovered that the immune system needs to be stimulated by exposure to pathogens in order to activate a type of immune cell, the regulatory T cell. This cell can repress the cells that overact in allergies and autoimmune disease. If the regulatory T cell is not adequately stimulated, the body will be more susceptible to both autoimmune diseases and allergies (Rook, G.A. The hygiene hypothesis and the increasing prevalence of chronic inflammatory disorders.Trans R Soc Trop Med Hyg (2007), pp1072-4).
There are of course some obvious exceptions to this theory. Hospitals where there are bacteria resistant to antibiotics (e.g. methicillin-resistant Staphylococcus aureus or MRSA), are reminders of the need to strike a balance. Cleanliness is imperative in a hospital environment, when illness is prevalent and the immune system of any patient may be suppressed and vulnerable to attack.
In the developing world, allergies and autoimmune diseases are rarer than in developed countries. The immune system is too busy fighting off dangerous pathogens, those organisms that cause malaria, AIDS, tuberculosis, schistosomiasis and leishmaniasis. The trade-off here is that many of these diseases can be, and often are, fatal.
Scientists are now trying to find ways to stimulate the regulatory T cell without subjecting people to these highly damaging infections (Zaccone, P. Schistosoma mansoni egg antigens induce Treg that participate in diabetes prevention in NOD mice. Eur J Immunol (2009), pp 1098-107).
Sources
Rook, G.A. Review series on helminths, immune modulation and the hygiene hypothesis: the broader implications of the hygiene hypothesis, Immunology (2009), pp 3-11.
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