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Though atopy (allergic disease) is known to be caused by genetic factors, environmental factors clearly play a crucial role as well. A person cannot become allergic to an allergen before the first environmental exposure to that allergen or to a chemically similar one. Genes only predispose individuals to developing allergies; the actual development of atopy always requires an environmental exposure.
Vercelli (2008) described the interaction between genes and the environment as "intricate." These interactions are made even more complex by the fact that they are affected by the developmental stage of the patient; atopic disease is associated with "critical developmental windows" (Vercelli 2008).
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Why Are Allergies Becoming More Prevalent?
Atopy is becoming more prevalent in developed nations. There is no good genetic explanation for this phenomenon; it must be due to environmental factors (Janewya 2001). Possible candidates for environmental causes have been proposed:
- decreased exposure to infectious diseases in early childhood
- increased environmental pollution
- changes in allergen levels
- changes in diet
Of these, the first is the best supported. Several studies have shown negative correlations between early childhood disease and allergy — in other words, if a person suffers a disease such as measles, that person is less likely to become atopic later in life (Janewya 2001).
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The Hygiene Hypothesis
The fact that exposure to disease seems to have a protective effect on the development of hay fever, eczema, and asthma has led to a model of allergy etiology called the "hygiene hypothesis." According to this hypothesis, the immune system lacks for legitimate targets as a result of the clean, hygienic environments in modern developed nations. Instead, this potent defensive weapon fixates on relatively harmless substances, causing allergies (Yamasaki et al. 2004; Willett 2000).
One piece of evidence for the hygiene hypothesis is the observation that exposure to the agent that causes tuberculosis, a mycobacterium, is negatively associated with the development of asthma (Yamasaki et al. 2004). The hygiene hypothesis may also explain the increasing prevalence of autoimmune disorders, not just atopic disorders (Willett 2000).
While basic disease-prevention measures such as handwashing and food safety practices should always be used, excessive or obsessive cleanliness may actually have a deleterious effect on health. The immune system evolved amid a certain amount of antigens, and the hygiene hypothesis suggests that these antigens are necessary to keep it from malfunctioning.
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- Janewya, Charles; Travers, Paul; Walport, Mark; and Shlomchik, Mark, 2001. Immunobiology: Fifth Edition. New York and London: Garland Science.
- Vercelli, D., 2008. "Advances in asthma and allergy genetics in 2007." Journal of Allergy and Clinical Immunology 122(2):267-271
- Willet, Edward, 2000. "The Hygiene Hypothesis." Edward Willett's Intergalactic Library.
- Yamasaki, A.; L. Cheng, S. Fukuda, M. Chinami, D. Fujita, D. Wasserman, and T. Shirakawa, 2004. "Allergic disorders: A model for establishing how to prevent common disease." Allergology International 53:61-68.