Why Are Allergies on the Rise?

When people at parties find out I’m an Allergist, I tend to get a lot of questions.  These range from the ordinary to the bizarre, but the one I hear most often is, “Why are allergies on the rise?” Science has been struggling to answer this since at least the 1970s, but in the past few years some major pieces of the puzzle have finally fallen into place.

To understand why allergies happen and how to treat them, we have to first understand a bit about the immune system. Its job is to figure out which of the thousands of things we are exposed to every day are dangerous and which are harmless, and then keep the bad ones (harmful bacteria, toxins, etc.) from doing us harm. Parts of our own body and parts of the environment that pose no harm are ignored, which is called tolerance. This system works amazingly well.  You don’t even notice it, because it’s working the way it’s supposed to.  However, when this system breaks, the results can be dramatic.

If parts of our own body are recognized as foreign or dangerous, it can lead to autoimmune diseases like lupus, rheumatoid arthritis, or Crohn’s disease. If our immune system fails to recognize harmful bacteria or viruses, this is an immune deficiency,  which can lead to more frequent or serious infections. And when harmless parts of our environment get recognized as foreign, this leads to allergies. So, allergies are, at their most basic level, a loss of immune tolerance.

Allergies have been around for as long as there have been people. Over the past 50-100 years, however, more and more people seem to be developing allergies.  Maybe people didn’t notice their hay fever as much when they were worried about the plague or being invaded by barbarians, but it does seem that allergies are indeed on the rise. Between 10-30% of people suffer from hay fever, 5-8% have food allergies, and around 60% of kids with asthma have an allergic component contributing to their disease. All told, about 50 million American suffer from allergies (1).

Many explanations for this have been suggested. Is it the food we eat? Cleaner living? Antibiotics? Pollution? All of these are probably partly correct , but there is no single factor that is driving this rise.

The most talked about theory is the Hygiene Hypothesis, which has been around since the late 1970s. Scientists noticed that allergies are more common in developed countries but less common in the developing world.  It turns out the parts of our immune system that cause allergies were designed to fight parasites like intestinal worms back in the Bad Old Days. Now that we have clean drinking water and modern sewage systems, this type of worm infection is increasingly rare in the developed world. Therefore, the theory seems to suggest our immune system gets “bored” and starts attacking things that aren’t dangerous, causing allergies.

Even though the rise in allergies seems to follow the drop in worm infections, that last part of the theory bothered me for years, because randomly attacking things is not how our bodies work. Fighting stuff takes energy, and our immune system is designed to be as efficient as possible – it doesn’t want to spend energy if it doesn’t have to. This is why we need booster shots every few years for some immunizations: the body recognized the original injection and made the appropriate antibody against it, but if those antibody producing cells don’t get activated occasionally, our immune system eventually gets rid of them because maintaining them uses energy that could be directed somewhere else.

Two years ago, scientists found a better explanation for why the Hygiene Hypothesis seems true (2). It turns out that those parasites were keeping part of our immune system, the part that causes allergies, in check. When they get into the intestine, parasites want to stay there and not get killed, so they send out chemical signals that push our immune system towards tolerance, in essence saying “Leave me alone.”  Over eons, our immune system pushes back, so it can kill them. Then, a few hundred years ago, worm infections dropped off with the introduction of modern sanitation and the system gets knocked out of balance.

The hygiene issue doesn’t end with worms. Our bodies are covered, inside and out, with bacteria, which collectively are called the microbiome (3). In fact, bacterial cells outnumber human cells 10 to 1 in our bodies. The vast majority of these are the “good bacteria” that do important things like help us to digest our food. Research in the past two years suggests that some of these bacteria have developed signals similar to those found in worms that push our immune system towards tolerance. Unfortunately, the balance of these bacteria has also been disrupted by things like soap, clean water and antibiotics, which may further increase the risk of allergies.

What about the foods we eat? Peanut allergy is far less common in China and in other parts of the world where it is primarily eaten in a boiled form. It turns out that boiling peanuts causes the proteins that cause allergies to clump together, making it less likely that our immune system will see them when we eat them.  On the other hand, dry roasting does the opposite, increasing the amount of allergy causing protein our bodies see when we eat them (4).  The same cannot be said for the other major foods that cause allergies, yet food allergy continues to rise.

What about changes in our environment? Several studies over the past five years have shown that climate change and rising carbon dioxide levels actually increase the amount of pollen and the ability of that pollen to cause allergies in plants like ragweed (5). Likewise, air pollution like diesel fumes and small particles cause inflammation in our noses and lungs and make them more likely to develop into allergies(6).

In the end, it seems that many aspects of our modern lives are working together to make allergies more common. What’s the solution? Should we give up running water? Infect ourselves with worms? (It has been tried, and doesn’t seem to help with allergies…seriously) (7).

As allergists, we have been able to help push the immune system back toward tolerance for hay fever and asthma symptoms for years. This is called immunotherapy, and works by slowly reintroducing the things someone is allergic to until their body no longer recognizes them as dangerous. Right now, there are also a number of promising new therapies being researched for food allergies, including possible immunotherapy, allergy blocking antibodies and even extracts based on traditional Chinese medicine.  Finally, through the study of the microbiome, we may be able to identify the bacteria that are most protective and take them as a daily supplement, as many people take probiotics today.

The more we learn about the immune system and how it works, the better we get at translating that to improve the quality of people’s lives. Because at the end of the day,  I want my patients to not only have their cake, but be able to eat it, too.

1. Allergy Statistics | AAAAI [Internet]. aaaai.org. [cited 2013 Mar 4]. Available from: http://www.aaaai.org/about-the-aaaai/newsroom/allergy-statistics.aspx

2. Hussaarts L, van der Vlugt LEPM, Yazdanbakhsh M, Smits HH. Regulatory B-cell induction by helminths: implications for allergic disease. J Allergy Clin Immunol. 2011 Oct;128(4):733–9.

3. Fung I, Garrett JP-D, Shahane A, Kwan M. Do bugs control our fate? The influence of the microbiome on autoimmunity. Curr Allergy Asthma Rep. 2012 Dec;12(6):511–9.

4. Kim J, Lee JY, Han Y, Ahn K. Significance of Ara h 2 in clinical reactivity and effect of cooking methods on allergenicity. Ann Allergy Asthma Immunol. 2013 Jan;110(1):34–8.

5. Wayne P, Foster S, Connolly J, Bazzaz F, Epstein P. Production of allergenic pollen by ragweed (Ambrosia artemisiifolia L.) is increased in CO2-enriched atmospheres. ANAI. 2002 Mar;88(3):279–82.

6. Bernstein DI. Diesel exhaust exposure, wheezing and sneezing. Allergy Asthma Immunol Res. 2012 Jul;4(4):178–83.

7. Bager P, Arnved J, Rønborg S, Wohlfahrt J, Poulsen LK, Westergaard T, et al. Trichuris suis ova therapy for allergic rhinitis: a randomized, double-blind, placebo-controlled clinical trial. J Allergy Clin Immunol. 2010 Jan;125(1):123–30.e1–3.