You've never had a problem with cats. You've lived in the same home for years. Spring pollen never bothered you.
Then, suddenly, you're sneezing every morning, your eyes are itchy, or you find yourself reaching for an antihistamine before visiting a friend with a pet.
It can be easy to assume that something else must be going on. After all, how could you suddenly become allergic to something you've been around for years?
The answer is that allergies can develop later in life. Your immune system can become sensitized to allergens over time, meaning a substance that previously caused no noticeable reaction can eventually trigger one. Allergies can also disappear for years and return later.
And while there isn't a rule that says you'll develop a new allergy every few years, changes in your immune system and your environment can make new sensitivities appear at different points throughout life.
Your immune system can change its mind
An allergy starts when your immune system mistakes a normally harmless substance for something dangerous.
During a process called sensitization, your body is exposed to an allergen and begins producing allergen-specific IgE antibodies. You may not experience obvious symptoms during that first exposure. But after sensitization occurs, another encounter with the allergen can trigger an immune response — including sneezing, congestion, itching, watery eyes, hives or other symptoms.
That means you don't necessarily have to be allergic to something the first time you encounter it.
Over time, repeated exposure can contribute to sensitization. Cleveland Clinic allergists note that new allergies can develop over a period of several years, rather than appearing immediately after the first exposure.
This is one reason someone can spend years around cats before eventually developing symptoms around them.
Moving can change your allergy triggers
One of the biggest reasons new environmental allergies can appear is a change in where you live.
Every region has its own mix of trees, grasses, weeds, molds and other environmental allergens. If you move somewhere with plants or other allergens you haven't encountered as much before, your immune system may eventually become sensitized to them.
For example, someone who never experienced significant seasonal allergies in one part of the country may move somewhere with a very different pollen profile and begin experiencing symptoms during a particular season.
It doesn't necessarily mean the new location is "causing" an allergy. Instead, you've changed the collection of allergens you're exposed to.
Your pets can become a new problem, too
Pet allergies can be particularly confusing because they don't always follow the logic people expect.
You can love cats or dogs, own one for years and still eventually develop symptoms around them.
Pet allergies are usually caused by proteins found in substances such as dander, saliva and other animal secretions — not simply the animal's fur itself. Environmental exposure can also vary depending on the animal, household and amount of contact.
So if you've recently noticed congestion, sneezing, itchy eyes or other symptoms around a pet that never bothered you before, it doesn't necessarily mean you're imagining the change.
Pet allergies can develop in adulthood, just like other environmental allergies.
It isn't always a new allergy
There is another possibility people often overlook: the symptoms may not actually be caused by an allergy.
A stuffy nose, watery eyes, coughing or irritation can have several possible causes. Non-allergic nasal conditions, infections and environmental irritants can sometimes produce symptoms that look similar to allergies.
This is why assuming you've "developed an allergy" based solely on symptoms can be misleading.
Allergy testing can help determine whether your immune system is actually reacting to a particular allergen. Depending on your symptoms and medical history, an allergist may use skin testing, blood testing or other evaluation methods.
Why does this keep happening?
There isn't one explanation for why a person develops a new allergy later in life.
Genetics can make some people more susceptible to allergic conditions. Changes in the immune system, new environmental exposures and hormonal changes may also play a role. People with conditions such as asthma or eczema may also be more likely to develop allergies later in life.
And sometimes, there simply isn't an obvious explanation.
That's what makes adult-onset allergies so frustrating. You can do everything the same way you've always done it and still find yourself reacting differently.
The allergy you didn't have five years ago may be real
If you've suddenly started sneezing around your cat, getting congested every spring or developing symptoms in a dusty room, don't automatically assume you're imagining it because you've "never been allergic before."
Allergies can develop at any age, and previous tolerance doesn't guarantee that you'll never become sensitized to a particular allergen.
Pay attention to patterns.
Do your symptoms appear around a particular animal? At a certain time of year? After spending time in a particular room? Or whenever you're exposed to dust or mold?
Those patterns can provide useful clues. And if symptoms are persistent, worsening or difficult to explain, an allergist can help determine whether you're dealing with a new allergy or something else entirely.
Because when your body suddenly changes the rules, guessing at the reason isn't always the best way to find relief.