'Confusion in the immune system': How allergies can develop later in life

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 allergist Kelly Colas.

Allergies can rear their ugly heads at any time in life – sometimes well into adulthood. These issues emerge seemingly from nowhere. But as researchers learn more, they are identifying a number of strange and surprising ways to "catch" new allergies – from tick bites to hormonal swings.

The Covid 'on switch'

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, Curman explains. 

Respiratory syncytial virus (RSV) – 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 physiologic example of being in the wrong place at the wrong time, 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'."

Confusion in the immune system

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.

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, 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 Colas, who is 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, Colas says. There's also a phenomenon known as "pregnancy rhinitis," which is when a pregnant person who has never had allergies before develops all the telltale symptoms, like sneezing, congestion and runny nose.

Pregnancy........

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