New allergies in adulthood

For Lisa, eating shellfish wasn't just a gastronomic delight — it was a central part of her family culture. Summers always brought the 40-something woman to Maine, where annual reunions were resplendent with platters of shrimp, crab, clams, and lobster. But one year, Lisa's beloved shellfish didn't love her back.

As she jubilantly dug into the feast, alarming symptoms arose: Lisa's lips and tongue swelled, while angry hives erupted all over her skin. She also vomited. Relatives called an ambulance, and Lisa was treated for anaphylactic shock. Tests confirmed her savored shellfish had turned on her: she was now highly allergic, after a lifetime without a problem.

Well into adulthood, most people believe they're past the possibility of developing a new allergy. But the phenomenon isn't unusual — and it happens more often among women, a Harvard specialist says.

"Most people think you develop allergies as a baby and they stick with you until adulthood, or that you outgrow them. But you really can develop an allergy at any time," says Dr. Camellia Hernandez, clinical director of the Division of Allergy and Clinical Immunology at Harvard-affiliated Brigham and Women's Hospital. "It's one of the most surprising things I tell people."

Allergy basics

Allergies come in many forms. They arise when the immune system mistakes a harmless substance — such as a food, pollen, animal dander, dust mites, or medication — for a dangerous intruder. After that, every time the body encounters that allergen, it mounts a reaction. Depending on the allergy and its severity, symptoms can include sneezing, watery eyes, coughing, itchiness, hives, vomiting, and trouble breathing.

Nearly one-third of American adults report having seasonal or food allergies, according to the CDC. But the seasonal variety — driven by pollen and often called hay fever — accounts for a far higher proportion over all.

Food foibles

Scientists are learning more about the kinds of allergies apt to crop up as we get older. Food allergies are far more likely to newly emerge in adulthood than seasonal allergies. Shellfish tops the list of late-blooming food allergens in adults, according to a 2018 survey of more than 40,000 adults published in Annals of Allergy, Asthma and Immunology.

The survey also indicated that nearly half of people who already had a food allergy developed at least one new one in adulthood. Remarkably, about one-quarter didn't have any food allergies as children. Trailing shellfish in the league of adult-onset food allergies are those to milk, wheat, tree nuts, and soy.

Women are more likely to develop food allergies in adulthood than men — and among people who do, women are more likely to grapple with severe food-related anxiety, according to a study published online Feb. 7, 2024, by Clinical and Experimental Allergy.

But why some people develop allergies, whether as a child or later in life, and others never do is still somewhat mysterious. It's likely due to a complex interaction between genes and environment, Dr. Hernandez says.

It appears, however, that the less you're exposed to a potential allergen — such as shellfish, which many people eat only a few times a year — the more likely it will later emerge as a new problem.

Vanishing allergies

There is, at least, some balance to the equation. If we can "grow into" allergies as decades pass, we can also — if we're lucky — grow out of them as well. For some people, an allergy that dogged them for years can suddenly cease to be a problem. This phenomenon is probably due to that fact that our immune system response quiets in midlife, meaning an allergen that once provoked a response now goes unchallenged, Dr. Hernandez says.

Some forms are more likely than others to cause less symptoms or even vanish. For example, about 30% of people allergic to peanuts eventually outgrow the threat, she says. And while anyone can develop an allergy later in life, those with a long history of seasonal allergies tend to have fewer symptoms as years pass, allowing those beleaguered by sneezing, runny nose, itchy eyes, and fatigue to greet warmer months with less dread.

Troubling signs

Our ability to readily spot an emerging allergy depends in large part on the type of allergy at play. It probably won't be immediately obvious if an environmental trigger, such as animal dander, suddenly becomes a problem, Dr. Hernandez explains.

For instance, a meet-and-greet with your friend's new kitten may be unexpectedly fraught. You leave sneezing repeatedly, with a runny nose and itchy, watery eyes, but think you're developing a cold. Back home, your symptoms remain for just a few hours and then retreat. That's a tip-off: if it were a cold, symptoms would last for several days or longer.

"You're not going to be able to know the first time," she says. "It usually takes a few instances of being with the animal before you realize what's happening."

New food allergies, however, may arrive with more of a bang. Before Lisa's life-threatening reaction to shellfish, she experienced subtle, easily dismissible signs of the brewing allergy. A few months prior to her emergency episode, Lisa vomited after eating shrimp scampi. "She didn't think anything of it — she thought it was food poisoning," Dr. Hernandez says. Soon after, however, devouring a lobster roll produced the same effect.

"A food allergy will become predictable: every single time you eat that food, you'll have those symptoms, and they will occur within minutes," Dr. Hernandez says.

Safe approaches to diagnosis

The pressing question becomes how to respond to these signs. That, too, depends on the type of allergy, Dr. Hernandez says.

With environmental allergies, such as to pollen or pet dander, you can try a do-it-yourself approach to relief by using a steroid-based nasal spray such as fluticasone (Flonase) or taking over-the-counter antihistamines such as cetirizine (Zyrtec), fexofenadine (Allegra), or loratadine (Claritin). If the remedies work, then you at least have a sense of what's occurring and how to relieve your symptoms. Tell your primary care doctor what you suspect.

But an allergist is key to diagnosing a potential new food allergy —something you shouldn't undertake yourself. An allergist can use a variety of tests to accomplish this. You may undergo a food challenge in an allergist's office — a test during which you consume a controlled quantity of the item in question as the doctor observes. If you experience an extreme reaction, she can intervene.

"An allergic reaction to food can sometimes be very severe, so it's not safe to self-diagnose," Dr. Hernandez says.

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