Why allergies can strike at any age

We often think of allergies as the result of an unlucky hand dealt in childhood, or even before birth. But any person, at any point in their life, can develop a new allergy, says the allergist Kelly Colas. Allergies can rear their ugly heads at any time in life, sometimes well into adulthood, and these issues emerge seemingly from nowhere. As researchers learn more, they are identifying a number of strange and surprising ways to catch new allergies, from tick bites to hormonal swings.

Some people are born with a genetic predisposition to allergy. But it's not all genetics, says Ruchi Gupta, director of the Center for Food Allergy and Asthma Research at Northwestern University in Illinois. Often, something is turning it on. One such on switch is Covid-19, which can leave its stamp on the immune system in the form of new allergies. A 2026 study found that catching Covid-19 is associated with a significantly higher risk of developing allergic rhinitis, more commonly known as hay fever, and asthma. Infection increased the risk, and vaccination decreased the risk, explains Philip Curman, a co-author of the study and a dermatologist at the Karolinska Institutet in Sweden. The immune system can run wild during a case of Covid-19, cranking up inflammation, including a type known to be related to allergic disease. Respiratory syncytial virus, a common childhood illness, and rhinovirus, a cold-causing virus, seem capable of similar effects, at least in children, previous studies show. Developing allergies in the wake of these illnesses is sometimes collateral damage, a physiological example of being in the wrong place at the wrong time, Dr Gupta explains. If your body sees a type of food or an environmental allergen coming in when your immune system is already overwhelmed by fighting off a virus, she says, your immune system's like, okay what is this now? Just get rid of everything.

Ironically, the treatments used to address some common illnesses may similarly have unintended effects. Antibiotic use, for example, has been linked to a subsequent risk of allergies. Dr Gupta, who has studied factors that may trigger allergies, says this could be because antibiotics disrupt the delicate balance of the gut microbiome, the massive colony of bacteria that live in the human gastrointestinal tract. Scientists still have a lot to learn about the gut microbiome, but it seems to play a role in myriad aspects of our health, including immune system function. Disrupting the microbiome, Dr Gupta says, may lead to confusion in the immune system, causing what should be harmless substances to be filed under threats that need neutralising.

Pregnancy is another seismic disruption to the immune system, and one that can have unexpected effects on allergies, says Dr Colas, an allergist at the University of Washington specialising in allergy management during pregnancy. About a third of women with pre-existing environmental allergies find that they get worse after conceiving, she says. There is also a phenomenon known as pregnancy rhinitis, when a pregnant person who has never had allergies before develops all the telltale symptoms, such as sneezing, congestion and a runny nose. Pregnancy rhinitis probably isn't a true allergy, but rather a temporary condition that results in allergy-like symptoms, Dr Colas says. One key point of differentiation is that symptoms usually disappear after delivery rather than lasting forever. Why it happens is really poorly understood, she says, but it seems related to hormonal fluctuations. The female reproductive hormones oestrogen and progesterone are well known to interact with the immune system. Research also suggests women may experience new allergies after they hit menopause, another time of great hormonal change. These hormones can do just about anything, Dr Colas says.

The bite of a lone star tick, an arachnid native to North America but spreading to new parts of the world as the climate warms, can be another surprising allergy trigger, specifically causing reactions to red meat. After a decadent dinner of lamb chops and French wine during a 2007 trip to London, Thomas Platts-Mills headed back to his hotel for a good night's rest. But then, he says, I woke up at 2:30 in the morning. By 3:00, I was covered in hives. Professor Platts-Mills, a professor of medicine and microbiology at the University of Virginia in the US, was uniquely qualified to understand this strange sequence of events. At the time of his trip, his laboratory was deep in the weeds of a medical mystery. His team was trying to understand why a string of patients in the US had developed allergies to red meat after years of omnivorous dining. By then, Professor Platts-Mills had a hypothesis: these bizarre allergies seemed to follow tick bites. His own case helped seal the deal. He had been covered with, and bitten by, larval ticks after hiking months earlier. Subsequent blood tests showed high levels of allergy-related antibodies. And now, his lamb dinner had him breaking out in hives. His unfortunate reaction helped his team define what is today known as alpha-gal syndrome. Ticks can carry in their saliva a sugar molecule called alpha-gal, which is produced naturally by most mammals, including those we eat. Humans, however, lost the ability about 10 million years ago, Professor Platts-Mills explains. That means that when a person is exposed to alpha-gal through a tick bite, their immune system may see it as a threat and respond accordingly. Everything non-primate is your enemy, he says. When that person encounters alpha-gal again, perhaps while enjoying a juicy hamburger, they may develop allergic symptoms including hives, swelling and gastrointestinal discomfort. Diagnoses of alpha-gal syndrome are rapidly becoming more common in the US as lone star ticks spread to new parts of the country, well beyond their original home in the Southeast. More than 90,000 people were diagnosed from 2017 to 2022 alone, according to government data.

While pregnancy rhinitis usually goes away once the baby comes, many people who develop sudden-onset allergies are not so lucky. Whether it emerges after an illness, a tick bite or seemingly out of nowhere, a new allergy may be permanent. Whether you are allergic to pollen or peanuts, dust or dogs, the same general process is to blame. Allergies happen when the body responds to what should be a benign substance as if it is a dangerous foreign invader and pumps out antibodies, a type of immune protein. When the immune system fires up, you are left with symptoms such as a runny nose, itchy eyes, or in serious cases even life-threatening anaphylaxis. Your immune system has recognised a substance as bad and created antibodies to whatever that is, Dr Gupta explains. Now, every time your body comes into contact with it, it goes on attack.

In cases where later-in-life allergies have a likely cause, such as inflammation related to an illness or gut microbiome changes after a course of antibiotics, there are clues about how to prevent or reverse the damage, at least in part. Dr Gupta, for example, recommends that people taking antibiotics also consume probiotics, prebiotics, yoghurt and fermented foods, all of which can help support the health and diversity of the gut microbiome. Preventing inflammation from raging out of control during a viral illness could also feasibly stop downstream consequences such as allergies, Dr Curman says. Could we affect the development of these secondary disorders, like asthma, with early treatment? he asks. That is an open question, but one worth investigating further. Immune-modulating drugs, in general, are promising for novice and veteran allergy sufferers alike. Xolair, a drug that has been used for decades to treat asthma and has more recently proven effective at taming food allergy reactions, works by targeting the immune system, blocking it from mounting an overly aggressive response that could become dangerous or fatal. Researchers are also studying other ways to retrain the immune system to overcome allergies. These experimental approaches include vaccines that may teach the system to respond to allergens in a more restrained way, just as vaccines prepare the body to fight off viruses without going haywire, and a single-dose treatment that aims to supply the body directly with allergen-blocking antibodies. As of now, there is no surefire way to reverse a new allergy, Dr Gupta says. But the future looks bright. I've been studying food allergies for 20 years, she says, and it's just such an exciting time to see so much progress.