And is there anything they can do to prevent them?

A man wearing a purple shirt with his back slightly turned; a light beige bubble swirls around him containing various allergenic elements, including shellfish, milk, cheese, nuts, a pea pod, a white dog with brown spots, a bee, a gray cat, pills, laundry detergent and an egg. On the bottom are pink and yellow flowers and a tree. The background is a light mustard color.
Credit…Kaitlin Brito

Hannah Seo

Lu Morales, 32, grew up eating a wide variety of Mexican seafood dishes. But at age 25, a takeout meal of shrimp egg rolls suddenly led to anaphylactic shock, an ambulance ride to the hospital and the diagnosis of a shellfish allergy.

Mx. Morales, who uses they/them pronouns, said the egg rolls caused coughing, wheezing and red, puffy eyelids. Now, Mx. Morales said, shellfish is completely off limits.

While most people won’t experience the gain — or loss — of allergies in adulthood, it is also not unusual, said Dr. Shradha Agarwal, an allergist and immunologist at the Icahn School of Medicine at Mount Sinai in New York City. Why allergies wax or wane, especially in adulthood, is largely not understood by scientists.

“There’s a lot of mystery in allergy,” Dr. Agarwal said.

Allergies come in many different forms, and generally develop when your immune system mistakenly treats a harmless allergen, like pollen or animal dander, as a threat, Dr. Agarwal said. It then reacts every time it encounters that allergen, with symptoms that can vary widely, ranging from coughing, sneezing and itchiness to more serious reactions like hives, vomiting, trouble breathing and loss of consciousness.

About 26 percent of adults and 19 percent of children in the United States have a seasonal allergy, and about 6 percent of adults and children have a food allergy, according to the Centers for Disease Control and Prevention.

The causes of allergies are complex, said Dr. Corinne Keet, a professor of pediatric allergy immunology at the University of North Carolina School of Medicine — depending on your genes and what kinds of allergens you’re exposed to and when.

But experts think that, in general, things that disrupt your immune system — like puberty, pregnancy, transient or chronic illnesses, or organ transplants — “can change your allergic responses to things that you previously tolerated,” Dr. Keet said.

Experts don’t know how common it is for different kinds of allergies to develop in adulthood, said Dr. Ruchi Gupta, a professor of pediatrics who specializes in allergy at the Northwestern University Feinberg School of Medicine.

Though we do have some data as it relates to food allergies. In one survey of more than 40,000 adults in the United States published in 2018, for instance, Dr. Gupta and her colleagues found that about 45 percent of those who had food allergies developed at least one new food allergy in adulthood. Of this group, a quarter never experienced food allergies as children.

An important question for researchers, Dr. Gupta said, is what exactly might cause adults to develop an allergy to a food they’ve eaten before. Right now, she said, we don’t know.

Dr. Jyothi Tirumalasetty, a practicing allergist and clinical assistant professor of medicine at Stanford University, has seen patients of all ages develop various kinds of new allergies, including some to common allergens such as pollen, pet dander or tree nuts.

“Losing” an allergy, or becoming “desensitized” to an allergen, happens frequently, Dr. Tirumalasetty said, especially beginning around (or after) middle age. Our immune responses “quiet down,” becoming weaker and less vigorous as we age, she said.

Some allergies are more likely to “resolve” than others, Dr. Keet said. Most penicillin allergies disappear over time, and seasonal allergies tend to lessen as you age, she said.

And while it’s much less common to grow out of certain food allergies such as those to tree nuts, fish and shellfish, Dr. Gupta said, an estimated 50 percent to 80 percent of children with milk or egg allergies grow out of them by age 10.

One common way people discover environmental allergies is by moving to a new area and encountering pollen they’ve never been exposed to before, Dr. Agarwal said. This wouldn’t technically be a “new” allergy, she said, but it’s a distinction that can make research in this area challenging. Similarly, moving away from such areas can lead to relief.

Molly Thessin, 30, who grew up near Nashville, said she had year-round pollen allergies as a child and had to take antihistamines regularly to relieve her symptoms. That all changed when she moved to Dallas at 23.

“I stopped taking allergy medicine for the first time in my life, and I was completely fine,” Ms. Thessin said.

A few years later, she moved to New York City, where she currently lives, and the allergies returned. It turns out she is allergic to most of the trees and plants in the Northeast, as well as to cats, dogs, mold and cockroaches.

As for whether there is anything adults can do to avoid developing new allergies, Dr. Agarwal said, experts don’t have the answer.

The only allergy prevention research right now is focused on preventing food allergies in children, Dr. Gupta said, which has little to do with preventing new allergies in adults.

At the end of the day, Dr. Keet said, you can’t really control whether you develop a new allergy as an adult. So, she said, “I wouldn’t worry about it.”

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