We often think that allergies are a predestined bad luck in childhood or even before birth.
However, allergist and immunologist Dr Kelly Colas said: "Anyone can develop new allergies at any stage in their life."
Allergies can appear suddenly at any time in life, sometimes even well into adulthood.
These problems appear seemingly out of nowhere, but as researchers learn more, they're discovering some strange and unexpected ways people might develop new allergies, from tick bites to hormonal fluctuations.
The “switch” of the new coronavirus
Some people are born with a genetic predisposition to allergies. But Ruchi Gupta, director of the Center for Food Allergy and Asthma Research at Northwestern University, Illinois, said, "It's not all genetic." Many times, "something turns it on."
One such "switch" is COVID-19. Infection with COVID-19 may leave a mark on the immune system, leading to new allergies.
A 2026 study found that infection with the new coronavirus is associated with a significantly increased risk of subsequent allergic rhinitis (commonly known as "hay fever") and asthma.
Dr Philip Curman, a dermatologist at Sweden's Karolinska Institutet and one of the co-authors of the study, explained: "Infection (with the new coronavirus) increases the risk, while vaccination reduces the risk."
He said that when infected with the new coronavirus, the immune system may overreact and trigger a lot of inflammation, including a type of inflammatory response known to be associated with allergic diseases.
Past research has shown that respiratory syncytial virus (RSV), a common childhood illness, and rhinovirus, which causes colds, appear to have a similar effect, at least in children.
Gupta explains that allergies that follow these illnesses sometimes feel like collateral damage, a physical example of "being in the wrong place at the wrong time."
"If your immune system is already overloaded with fighting viruses, and you encounter a certain food or environmental allergen, your immune system might think, 'Now what is this? Just get rid of it all,'" she says.
"Immune system confusion"
Ironically, some of the methods used to treat common illnesses can also have unintended consequences. For example, antibiotic use has been found to be associated with the risk of later developing allergies.
Gupta has studied factors that may trigger allergies. One reason may be that antibiotics disrupt the delicate balance of the gut microbiome, she said. Gut microbiota is a large community of bacteria living in the human gastrointestinal tract.
Scientists still don't know much about the gut microbiome, but it appears to be involved in many aspects of human health, including immune system function.
Gupta said disrupting the gut microbiome could cause "chaos in the immune system," causing otherwise harmless substances to be misclassified as threats that must be eliminated.
Pregnancy is also a major change to the immune system and can have some unexpected effects on allergies. Colas, an allergist and immunologist at the University of Washington who specializes in allergy management during pregnancy, said that about one-third of women who already have environmental allergies will find that their symptoms worsen after pregnancy.
There is also a phenomenon called "pregnancy rhinitis," in which pregnant women who have never had allergies suddenly develop typical allergy symptoms, such as sneezing, nasal congestion, and runny nose.
Pregnancy rhinitis is likely not a true allergy, but a temporary condition that produces allergy-like symptoms, Colas said.
What is one of the important differences? These symptoms usually go away after delivery rather than persisting.
Why this occurs is "really still very unclear," Colas said, but it appears to be related to hormonal fluctuations. After all, the female reproductive hormones estrogen and progesterone have long been shown to interact with the immune system.
Research also shows that women may develop new allergies after entering menopause, another stage of dramatic hormonal changes.
"These hormones can do almost anything," Colas said.
Allergy risks hidden in grass
The lone star tick is native to North America but is spreading to other parts of the world as the climate warms. Bites from this arachnid can also be another surprising trigger of allergies, especially to red meat.
In 2007, during a trip to London, Thomas Platts-Mills enjoyed a sumptuous dinner of lamb chops and French wine, and then returned to the hotel to get a good night's sleep.
"I woke up at 2:30 in the morning. By 3 o'clock, I was covered in hives," he said.
Platz-Mills, a professor of medicine and microbiology at the University of Virginia, had just the expertise needed to understand this strange situation.
At the time, his lab was delving into a medical mystery. His team was trying to understand why a group of people in the United States who had been eating all kinds of meat for years suddenly became allergic to red meat.
Platz-Mills has hypothesized that this strange allergy appears to occur after a tick bite. And his own experience further supports this idea.
A few months ago, he was covered in tick larvae and bitten after a hike. Blood tests later showed high levels of allergy-related antibodies. Now, a mutton dinner caused hives all over his body.
Platz-Mills' unfortunate allergic reaction helped his team define what is today known as alpha-Gal syndrome (red meat allergy).
Tick saliva may contain a sugar molecule called α-Gal. Most mammals, including cows, pigs and sheep consumed by humans, naturally produce this molecule.
However, Platz-Mills explains, humans "lost the ability to produce it about 10 million years ago."
This means that when a person is exposed to alpha-Gal through a tick bite, their immune system may view it as a threat and respond accordingly.
"All non-primates become your enemy," Pratts-Mills said.
When the person is later exposed to alpha-Gal, such as by eating a juicy hamburger, they may develop allergic symptoms including hives, swelling, and gastrointestinal discomfort.
The number of people diagnosed with alpha-gal syndrome is increasing rapidly in the United States as the lone star tick spreads from its primary distribution area in the southeastern United States to other parts of the country.
U.S. government data shows that between 2017 and 2022 alone, more than 90,000 people were diagnosed.
Can sudden allergies be cured?
Pregnancy rhinitis usually resolves after the baby is born, but many people who develop allergies suddenly aren't so lucky.
Whether they appear after illness, a tick bite, or appear suddenly for seemingly no reason, new allergies can be permanent.
Whether you are allergic to pollen or peanuts, dust or dogs, the process behind it is roughly the same: the body treats substances that should be harmless as dangerous foreign invaders, and thus produces large amounts of antibodies, a type of immune protein.
When the immune system initiates a response, symptoms such as runny nose and itchy eyes may occur. In severe cases, it may even lead to life-threatening anaphylactic shock.
"Your immune system has identified a substance as harmful and developed antibodies against it. Then every time the body is exposed to it, it will attack," Gupta explains.
If allergies that appear in adulthood have a clearer cause, such as inflammation from an illness or changes in the gut microbiome after taking a course of antibiotics, research also provides some clues that the damage may be at least partially prevented or reversed.
For example, Gupta recommends that people taking antibiotics also consume probiotics, prebiotics, cheese and fermented foods, which can help maintain the health and diversity of the gut microbiota.
Researchers are also looking at other ways to retrain the immune system and allow the body to overcome allergies.
These still-experimental approaches include vaccines, which hope to teach the immune system to respond to allergens in a more restrained manner, just as vaccines prepare the body to fight viruses while preventing a runaway immune response.
Another type being studied is a single-dose therapy that aims to deliver antibodies directly to the body that can block allergens.
Gupta said there's still no 100 percent reliable way to reverse new allergies, but the future is promising.
"I've been studying food allergies for 20 years. It's a really exciting time to see so much progress," she said.
