What Causes Allergies?

The prevalence of allergies is on the rise, but why do some people suffer allergic reactions while others don't? If you were lucky enough to be an allergy-free kid, could you still develop allergies as an adult?

Sabrina Stierwalt, PhD
4-minute read
Episode #175

Almost 8% of adults and 9% of children in the U.S. suffer from nasal allergies or hay fever, and the symptoms result in over 11 million visits to the doctor annually. Three million people in the U.S. further report an allergy to peanuts, or tree nuts or both, including 8% of children, according to the American Academy of Allergy Asthma and Immunology.

The prevalence of allergies also appears to have increased over the last 50 years and shows no signs of slowing down. As the House Call Doctor explained, allergy symptoms occur when our immune system mistakes a normal substance, like pollen or a nut, for an intruder, and wages war against it through the release of antibodies. But why does this response happen in some people and not others? Why are allergies becoming more and more common? If you were lucky enough to be an allergy-free kid, could you still develop allergies as an adult?

Genetics and Our Environment

Scientists are not entirely sure why some people suffer from allergies while others live allergy-free, but studies show that genetics and environment are both important factors.

Studies of twins for which at least one twin is allergic to peanuts have found that, in the case of fraternal twins, the other twin has a 7% chance of also having the allergy. Among identical twins, however, both twins were allergic in 64% of cases. Thus, our genetics clearly influence whether or not we will have an allergy.

However, the strength of the connection between genetics and allergies is still up for debate. In a Canadian study of over 900 siblings of kids with peanut allergies, for example, only 1.7% of siblings were also allergic. We also appear to inherit the tendency to have allergies, but not a specific allergy. In other words, having a parent who is allergic to nuts does not mean that you will necessarily also be allergic to nuts, but it does increase your chances of having any allergy at all.

Our environment must also factor into determining whether or not we will ultimately be allergic. A recent study has shown that the rates of allergies and asthma are lower for those living on farms, possibly due to exposure to a specific molecule found in cow manure.

Ultimately, our environment determines whether or not we will be exposed to a particular allergen, a requirement for developing an allergy. However, it is not always clear whether exposure can be beneficial, because it offers your body the chance to build allergy-busting antibodies, or whether complete avoidance of an allergen for as long as possible will have better results. Thus, scientists still actively debate, for example, whether pregnant women should eat peanut butter so that the baby can learn in utero not to be allergic to the nuts, or whether babies do not yet have the strength to combat a potential peanut allergy and this early exposure is ultimately allergy-triggering.


Please note that archive episodes of this podcast may include references to Ask Science. Rights of Albert Einstein are used with permission of The Hebrew University of Jerusalem. Represented exclusively by Greenlight.

About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.