Allergies are often associated with childhood. However, they can appear at almost any stage of life. Experts say adults who have never experienced allergies can suddenly develop reactions to foods, pollen and other substances.
Allergist Kelly Colas says a person can develop a new allergy at any point in life. Researchers are now investigating why this happens. Their findings suggest that infections, medications, hormonal changes and even tick bites can alter how the immune system responds to normally harmless substances.
Viral Infections May Trigger New Allergies
Genetics can increase a person’s risk of developing allergies. However, researchers say genes are only part of the picture. Certain events may act as an immune system “on switch.”
Covid-19 is one possible trigger. A 2026 study found that people who had Covid-19 faced a higher risk of developing allergic rhinitis and asthma.
Philip Curman, a study co-author and dermatologist at Sweden’s Karolinska Institutet, said infection appeared to increase the risk. Vaccination, meanwhile, was associated with a lower risk.
Researchers believe intense inflammation during a viral infection may affect the immune system. Similar effects have been observed with respiratory syncytial virus and rhinovirus, particularly in children.
The theory is that an overwhelmed immune system may respond incorrectly when it encounters an otherwise harmless allergen.
Antibiotics and Hormones Can Change Immune Responses
Illness is not the only possible trigger. Antibiotics have also been associated with a later risk of allergies.
Ruchi Gupta, director of the Center for Food Allergy and Asthma Research at Northwestern University, says antibiotics can disturb the gut microbiome. This community of bacteria plays an important role in many areas of health, including immune function.
Changes in the microbiome may affect how the immune system identifies threats. As a result, harmless substances could potentially be mistaken for dangerous ones.
Pregnancy can also change allergy symptoms. Colas says about one-third of women with existing environmental allergies find that their symptoms become worse during pregnancy.
Some pregnant people also develop a condition called pregnancy rhinitis. It can cause sneezing, congestion and a runny nose in people who have not previously had allergies.
Pregnancy rhinitis is generally not considered a true allergy. Symptoms usually disappear after delivery. Hormonal changes are thought to play a role.
Menopause may also influence allergy patterns. Major changes in reproductive hormones can affect the immune system and may contribute to new allergic reactions in some women.
A Tick Bite Can Lead to a Meat Allergy
One of the most unusual causes of a new allergy involves ticks.
The lone star tick can trigger a condition known as alpha-gal syndrome. People with the condition can develop allergic reactions after eating red meat and other products containing alpha-gal.
Medical researcher Thomas Platts-Mills helped identify the connection. After being bitten by ticks, he experienced a severe reaction after eating lamb.
Researchers later found that lone star ticks can introduce a sugar molecule called alpha-gal into the human body through their saliva. Most mammals naturally produce alpha-gal, but humans do not.
The immune system can treat the molecule as a threat. Later exposure through foods such as beef, pork or lamb can then trigger symptoms.
Those symptoms can include hives, swelling and digestive problems. In severe cases, allergic reactions can become life-threatening.
US government data indicates that more than 90,000 people were diagnosed with alpha-gal syndrome between 2017 and 2022. The condition has become more widely recognised as lone star ticks expand into new areas.
Can New Allergies Be Reversed?
There is currently no guaranteed way to reverse a newly developed allergy. Some temporary conditions, such as pregnancy rhinitis, usually disappear after the underlying trigger ends.
For other allergies, the immune system can remain sensitised. It produces antibodies that recognise a normally harmless substance as a threat. Future exposure can then trigger symptoms ranging from a runny nose and itchy eyes to severe anaphylaxis.
Researchers are exploring ways to change this immune response. Some studies are examining vaccines and other treatments designed to make the immune system react more calmly to allergens.
Xolair, an immune-targeting medicine already used for asthma, has also shown benefits in treating certain food allergy reactions.
Scientists are continuing to investigate whether early treatment during infections could reduce later allergy risks. Researchers are also studying ways to restore a healthy gut microbiome after antibiotic use.
For now, there is no simple cure for sudden-onset allergies. However, growing knowledge about the immune system is giving researchers new possibilities for prevention and treatment.
