The days get longer, birds start chirping, and flowers start blooming. And then there’s the uproarious sneezing and nose-blowing of itchy, red-eyed humans cursed with allergies. Allergies seem fairly common, but why do some many people have such severe reactions to such harmless grains of pollen?
Allergies are the result of immune systems gone haywire and oversensitive. When the immune system patrols a person’s body, they use chemical signals on the surface of the object to determine if it is part of the body or if it’s foreign. For whatever reason, those with allergies have immune systems that interpret the harmless environmental substances as dangerous. When the allergen, such as pollen, is first introduced into the body, the immune system panics and interprets it as a dangerous invader. In response it starts mass-producing IgE antibodies, also known as the “allergy antibody.” These antibodies start floating throughout the bloodstream in a well-meaning attempt to protect the body against future “invasions” of the allergen, and the immune system is now “sensitized” – future exposure will lead to an immune response.
When the allergen is again detected in the body, the immune system kicks into high gear, responding as if the harmless pollen were some dangerous parasite or bacteria. The IgE antibodies bind to the allergen and activate the immune system’s mast cells, a type of white blood cell. The mast cell promptly explodes and releases tons of histamines and cytokines. These chemicals cause those familiar symptoms of allergies, resulting teary eyes, general itchiness, and runny noses.
But why do immune systems react so much to such mundane things? And why do some people seem perfectly fine when they enjoy a pollen-filled spring day?
Children who lack exposure to certain types of microbes are more prone to developing allergies.
David Strachan in 1989 published an article in British Medical Journal that formally proposed the “hygiene hypothesis,” the idea that exposure to infections decrease the risk of allergies. He noticed that hay fever and eczema were both less common in children from larger families, which he explained as having exposure to more pathogens. This hypothesis has since then been studied by immunologists and epidemiologists. While some findings have been conclusive, other findings have been contradicting.
The reasoning behind Strachan’s hygiene hypothesis lies in trends in rising incidences of allergies in industrialized nations. While allergies are heritable traits that are passed down, the rise in allergies has happened within such a short time frame that it cannot be solely explained by genetics. Immunologists have started to explore this hypothesis as a general idea that reduced exposure to microbes is resulting to a rise in allergies.
Recently this idea has been attributed to the higher “standards of personal cleanliness.” Reserachers have noted that the frequencies of allergies generally are lower in developing nations. The health researcher Dr. Erika Von Mutius examined children in East and West Germany, expecting to find that the children living in cleaner, richer, and healthier West Germany would have lower rates of allergies and asthma. What she found, instead, was that these children had higher rates of allergies and asthma compared to their counterparts living in East Germany. What was more surprising was after Germany was reunified, the rates of allergies and asthma of children in former East Germany increased significantly. These findings seem to indicate that children who lack exposure to certain types of microbes are more prone to developing allergies. While the exact mechanism is unclear, what is hypothesized is that these overly sanitized environments prevent a crucial stage in immune system development, rendering it unable to distinguish harmless substances from other, more life-threatening pathogens.
One of the clearest concrete examples of the hygiene hypothesis is the effects of the Hepatitis A virus. The research scientists Matricardi and others studied the proportion of Italian military students with allergies, and other scientists have since then replicated his studies with other populations. They have found that “independent of age, sibling size, birth order, area of residence” and several other factors, exposure to the Hepatitis A virus resulted in a significant decrease in the rate of allergies . The study of San Marino students found that among those who were infected by the Hepatitis A virus, the proportion that developed allergies was almost half of those who were not infected by the virus.
In light of Strachan’s hygiene hypothesis, should we disregard all of our higher “standards of cleanliness” that seems to be causing our allergies? Unfortunately, the evidence suggests this would not be wise. While there are clear links with the Hepatitis A virus and other studies, there is no clear consensus regarding links between other microorganisms and allergy rates. For example, there appears to be a relationship between wild measles infection and rate of allergies, but the “epidemiological evidence is conflicting,” with some results showing that measles infection may increase the rate of allergies.
In addition, the current hygiene standards has helped drastically lower mortality rates from several infectious and dangerous diseases such as cholera and tuberculosis. Even if the hygiene hypothesis was proved conclusively, the evidence suggests that allowing such exposure to pathogens would increase mortality rates- “if the main effect was, for example a reduction in hayfever, with little or no impact on asthma, the ‘trade off’ would represent a very poor bargain.”
Perhaps the best approach is to be pragmatic towards cleanliness. Knowing that a little dirt can be helpful, there is no need to be constantly sanitizing our hands every other minute of the day.