Almost all of us have, or know someone who has an allergy. An allergy is an overreaction of your immune system to something that is foreign to your body, but may not be dangerous to other healthy individuals. People can be allergic to pollen, mold, antibiotics, food such as peanuts or shellfish, or any number of other things in our environment. When exposed to an allergen, your immune system generates an inflammatory response, giving you symptoms like a runny nose, itch, or rash. Scratching and sneezing might be uncomfortable, however in other situations, an allergic reaction can be life threatening. Individuals with severe allergies can develop an “anaphylactic” reaction after exposure to an allergen, which can cause trouble breathing and a sudden drop in blood pressure, and often requires immediate medical attention.
Disturbingly, the incidence of allergies has been on the rise, with a dramatic increase in the United States in the past thirty years to the point where 1 out of every 5 Americans has an allergy [1]. The number of food allergies among children increased 18% between 1997 and 2007 [2]. Researchers have been trying to figure out why there has been such a sharp increase in the number of allergy sufferers, and have come up with several ideas.
The “Hygiene Hypothesis”
The data show that the number of those affected by allergies is not increasing uniformly worldwide. In fact, this rapid increase is not observed in developing countries, but only in developed nations. Within any particular country, allergies are more prevalent in cities than in rural areas [3]. This information tells us that the environment is playing a big role in determining if someone develops allergies. The prevailing hypothesis for this pattern of allergic disease is known as the “Hygiene Hypothesis.” Essentially, this hypothesis says that with improvements in sanitation, health care, and hygiene, we are reducing or even eliminating our exposure to many microbes and parasites. Our immune systems evolved to help us fight off pathogens. As a result of living in cleaner environments, however, we are exposed to less disease-causing organisms and our immune systems have less work to do. Some scientists believe that this causes the immune system to overreact to harmless things in our environment, like that peanut butter you just snacked on, or your friend’s cat.
A smoggy outlook?
In addition to improved hygiene, developed countries and our cities share another environmental factor, air pollution. The type of air pollution associated with burning petroleum-based fuels has been associated with an increase in allergic diseases [4]. There is also evidence that air pollution particles can interact with potential allergens like pollen. Pollen in heavily polluted areas has been found to contain more allergenic proteins than in less polluted areas [4]. The change in the number and type of these proteins can make them more likely to cause allergies.
However, over the past 30 years, there have been major improvements in air quality in the United States. The concentrations of all major air pollutants, including carbon monoxide, sulfur dioxide, and ozone, have decreased (http://www.epa.gov/airtrends/). Air pollution may contribute to an increased incidence in allergies or exacerbate the effects of allergens, but the decrease in air pollution did not result in a decrease in the number of allergy sufferers, indicating that there are other contributing factors.
In our genes
Your genetic background can predispose you to having an allergy. If one of your parents has an allergy, there is an increased chance that you will have an allergy. If both of your parents have allergies, there is an even higher chance that you will develop allergies. However, the changing prevalence of a trait due to genetics is a slow process and occurs over the course of generations, not as fast as within a decade.
While genetics might play a part, it cannot account for this rapid rise in allergies; environmental conditions probably play a larger role. Better sanitation, an increase in air pollution, and other environmental factors probably have varying contributions to the increase in allergies.
A wormy cure?
Recently, there has been a lot of research into the “Hygiene Hypothesis”, which is the idea that a lack of parasitic infections could cause an increase in allergic disease. Researchers have found that some parasitic worms might be able to regulate inflammatory diseases, like allergies. There are studies focusing on infections by helminths, a class of parasitic worms that includes hookworm, whipworm, and the trichina worm. Areas with high rates of helminth infection have been found to have low rates of allergic disease [3]. Several studies have used populations with high helminth infection rates to look at the effect of de-worming campaigns on allergy sensitization. Those individuals that were treated to cure them of their helminth infection were at an increased risk of allergen sensitization compared to individuals that were not cured of their helminth infection [5].
How does this happen? Scientists have shown that helminths can suppress the human immune system, and may be able to reduce the responsiveness of the immune cells. Helminth infection increases our production of a protein called interleukin-10, which signals to immune cells to reduce inflammation. Since allergies are caused by an over-responsiveness of immune cells, helminth infection might dampen the initial immune response and reduce the symptoms of an allergic reaction [6].
While this is an encouraging finding for allergy sufferers, this is not by any means a cure yet. Helminth infections have detrimental effects on humans, including vitamin deficiency and malnutrition. Decreasing the responsiveness of the immune system is also very risky and could make people more susceptible to pathogens. Despite this, trials are currently underway to evaluate the safety and efficacy of helminth infection as a treatment for allergies [6]. Once scientists figure out exactly how these worms regulate the immune system, they might also be able to design drugs to mimic their immune regulating effects without the side effects of a parasitic infection. A helminth-based therapy – either through direct infection or a drug mimicking infection – could eventually help to alleviate the suffering of many people with allergies.
Dana E. Christofferson
Harvard Medical School
References
1. Asthma and Allergy Foundation of America; Allergy Facts and Figures http://www.aafa.org/display.cfm?id=9&sub=30
2. Branum A, Lukacs, D. Food Allergy Among U.S. Children: Trends in Prevalence and Hospitalizations. NCHS Data Brief (2008) No. 10. http://www.cdc.gov/nchs/data/databriefs/db10.htm
3. Yazdanbakhsh M, Kremsner PG, van Ree R. Allergy, parasites, and the hygiene hypothesis. Science (2002) 296:490-4.
4. Bartra J, Mullol J, del Cuvillo A, Dávila I, Ferrer M, Jáuregui I, MontoroJ, Sastre J, Valero A. Air pollution and allergens. J Investig Allergol Clin Immunol (2007) Vol. 17, Suppl. 2: 3-8.
5. Flohr_C, Quinnellw RJ, Britton J. Do helminth parasites protect against atopy and allergic disease? Clinical and Experimental Allergy (2008) 39:20-32.
6. Can hookworms protect against allergies? By Elizabeth Svoboda. The New York Times. July 1, 2008. http://www.nytimes.com/2008/07/01/health/01iht-01prof.14122951.html?_r=1
Links of interest
WNYC RadioLab podcast “Parasites” http://www.radiolab.org/2009/sep/07/