Are allergies genetic?

Over 100 million people, and more than 30% of adults, suffer from some type of allergy in the United States, and that number is on the rise. But where are these allergies coming from? Do we inherit them, or do they result from environmental factors?

The answer involves a little of both, said Dr. Derek Chu, an allergy specialist at McMaster University in Ontario. Allergies occur when the immune system mistakes a harmless substance, such as pollen or a certain type of food, for a dangerous substance and attempts to get it out of the body. Once the body reacts this way toward an allergen, it will repeat the allergic response for every new exposure, unless there is a successful intervention.

"The body goes awry and is trained to do the wrong thing," Chu told Live Science.

According to Leah Kottyan, an immunologist at Cincinnati Children's Hospital, there are several main types of allergies. They include allergic dermatitis, which manifests through skin conditions such as rashes and hives; allergic asthma, which involves airway inflammation and the overproduction of mucus; allergic rhinitis, which results in sneezing and a stuffy nose; and food allergies, which can trigger a whole-body immune response. All of these responses could be driven by the same allergen, Kottyan noted. Moreover, people who have one type of allergic response are more likely to have others and more likely to be allergic to more than one thing.

According to Kottyan, there is almost certainly a genetic component to the development of allergies. Independent studies comparing the prevalence of allergies in identical and fraternal twins found that the identical twins were far more likely to exhibit similar allergies than the fraternal twins, indicating that their genetic makeup played a role. In one study, the identical twins had an average of 95% similarity between four different types of allergies, whereas fraternal twins had an average of around 37%.

However, the link between genetics and allergies is complicated. Allergic responses can be traced to mutations in hundreds of genes. One of the most studied such genes, known as filaggrin or FLG, affects the development of the skin's moisture barrier, Kottyan said. Mutations in FLG cause the moisture barrier to not act as it should. This increases a person's risk of developing eczema, allergic dermatitis and other types of allergies.

FLG mutations can put the skin in a compromised state, which can increase the skin's susceptibility in getting cuts and scrapes and from developing dry and cracked skin. When this compromised skin comes into contact with an allergen, the immune system is more likely to become sensitized improperly to that allergen, Kottyan said. This can cause an allergy to develop.

"Literally, when there's food on the baby's skin, the food is coming in through the damaged skin barrier, and the child is getting sensitized to that food through the skin," Kottyan told Live Science.

Environmental factors

Although allergic reactions have a genetic component, environmental factors — including exposure to different allergens — play a big role in how someone develops allergies.

"It doesn't track one to one," Chu told Live Science. In a recent study Chu co-authored, scientists investigated nongenetic risk factors for allergies, including the baby's method of delivery, early exposure to antibiotics, and when they were introduced to solid food.

Moreover, even people who inherit a tendency to develop allergies may not be allergic to the same things as their parents. Rather, whether the baby becomes allergic to peanuts, for example, will likely be determined by their early exposure to that allergen. There is now a FDA-approved immunotherapy treatment for children that centers on daily exposure to peanut proteins over a long period of time.

"If the immune system is exposed inconsistently, then it's not learning to tolerate what it's being exposed to, and instead it can easily end up misbehaving, leading to allergic reactions," Chu said.

The microbiome also plays a big role in the development of allergies. Scientists have found that early exposure to a wide variety of environmental microbes results in robust and healthy gut and skin microbiomes that are more tolerant of potential allergens. Without this diversity, or with a microbiome consisting of higher percentages of certain microbes, it is more likely that a person will develop conditions like eczema and food allergies.

A similar theory suggests that the eradication of many common parasites, and the more sterile environment many of us now live in, may have played a role in the uptick in allergy cases in the United States. With no parasites and fewer viruses and bad bacteria to go after, our immune systems may be developing too many allergy-fighting cells instead of the bacteria and virus-fighting ones.

Researchers like Chu and Kottyan are continuing to pinpoint the risk factors that can cause someone to develop allergies and test new preventive measures to help children grow into allergy-free adults. For now, Kottyan said, the best steps parents can take to prevent allergies in their children is to expose them to common allergens early and often and to take good care of their children's skin, especially in areas that are prone to dryness and eczema.

"Allergy genetics are not deterministic; they aren't going to say you absolutely are going to get a disease," Kottyan told Live Science. "It's not a life sentence."