Allergies happen because your immune system misidentifies a harmless substance, like pollen or peanut protein, as a threat and mounts a defensive response against it. Nearly one in three U.S. adults (31.7%) has at least one diagnosed allergic condition, so if you’re dealing with allergies, you’re far from alone. The reasons you specifically developed them come down to a combination of your genetics, your early childhood environment, and what your immune system has encountered over time.
What Actually Happens Inside Your Body
An allergic reaction is a two-stage process. The first stage is sensitization: the first time your immune system encounters an allergen (say, cat dander), it decides this substance is dangerous and produces a specific type of antibody called IgE. These IgE antibodies attach themselves to mast cells, which are immune cells packed with inflammatory chemicals and stationed throughout your skin, airways, and gut lining. This attachment is extremely strong, which is why even a tiny amount of IgE is enough to prime your tissues for a reaction, and why sensitization can persist for years.
The second stage is the reaction itself. The next time you breathe in cat dander, the allergen lands on those primed mast cells and bridges two neighboring IgE antibodies together, like a key fitting into a lock. That bridging triggers the mast cell to burst open and dump its contents, primarily histamine, into the surrounding tissue. Histamine is what causes the familiar symptoms: it dilates blood vessels (creating redness and swelling), irritates nerve endings (causing itching and sneezing), and increases mucus production (hello, runny nose). All of this happens within minutes, which is why allergic reactions feel so sudden.
Genetics Set the Stage
The tendency to develop allergies runs in families. If one of your parents has any allergic condition, your risk of developing one increases roughly 1.5-fold. If both parents are affected, the risk climbs to about 5-fold. What you inherit isn’t a specific allergy to, say, dust mites or shrimp. You inherit a general predisposition called atopy, which means your immune system is wired to overproduce IgE antibodies in response to common environmental proteins. Which specific allergens your body reacts to depends on what you’re exposed to over the course of your life.
This is why siblings raised in the same household can end up with completely different allergy profiles. One might get hay fever, another might develop a food allergy, and a third might have none at all. The genetic loading makes allergies more likely, but it doesn’t make them inevitable.
Your Childhood Environment Matters
Genetics alone don’t explain why allergies have surged over the past several decades. In the early 1990s, researchers proposed what’s now called the hygiene hypothesis: the idea that growing up in overly clean environments deprives the developing immune system of the microbial exposure it needs to calibrate properly.
Here’s the logic. Early in life, the immune system naturally leans toward the type of response that drives allergies (a Th2 response). Exposure to bacteria, viruses, and other microbes during infancy helps shift the immune system toward a more balanced state. Without enough of that microbial training, the immune system stays tilted toward allergic responses. This helps explain why children who grow up on farms, attend daycare early, or have older siblings tend to develop fewer allergies. They encounter more germs in their first years of life, which nudges their immune systems in a less reactive direction.
Why Allergies Can Start in Adulthood
If you didn’t have allergies as a kid but developed them later, you’re not imagining things. Adult-onset allergies are common and still not fully understood. The most likely explanation is a complex interaction between your genetic predisposition and new environmental triggers. Moving to a different region exposes you to unfamiliar pollen or mold species your immune system has never encountered. A viral respiratory infection can temporarily alter how your immune system responds to airborne proteins. Hormonal shifts during pregnancy or menopause can also change immune reactivity.
Interestingly, infrequent exposure to a potential allergen seems to increase the chance it will eventually cause problems. Shellfish is a good example: many people eat it only a few times a year, which may be just enough contact for the immune system to flag it as foreign without building tolerance.
Climate Change Is Making It Worse
If your seasonal allergies feel worse than they used to, they probably are. Rising temperatures, fewer frost days, and higher levels of carbon dioxide in the atmosphere are all affecting pollen. According to the CDC, these shifts are changing when allergy season starts and ends, how much pollen plants produce, and how potent that pollen is at triggering symptoms. The result is a longer, more intense allergy season in many parts of the country. Spring pollination is starting earlier, fall ragweed season is stretching later, and plants are generating more allergenic pollen overall.
When Pollen Allergies Affect Your Food
If you’ve ever noticed your mouth tingling or itching after eating certain raw fruits or vegetables, your pollen allergy may be the cause. This is called oral allergy syndrome, and it happens because proteins in some foods are structurally similar to pollen proteins. Your immune system confuses the two.
- Birch pollen allergy: can trigger reactions to apples, cherries, peaches, carrots, almonds, hazelnuts, and peanuts.
- Grass pollen allergy: can trigger reactions to peaches, celery, tomatoes, melons, and oranges.
- Ragweed allergy: can trigger reactions to bananas, cucumbers, melons, and zucchini.
These reactions are usually mild, limited to itching or tingling in the mouth, and they typically don’t occur when the food is cooked (heat breaks down the proteins your immune system is reacting to).
How Allergies Are Identified
If you’re unsure what you’re reacting to, allergy testing can narrow it down. The most common method is a skin prick test, where tiny amounts of suspected allergens are introduced just below the skin’s surface. A small raised bump within 15 to 20 minutes indicates a reaction. Blood tests that measure allergen-specific IgE levels offer an alternative, particularly for people who can’t stop taking antihistamines or who have severe skin conditions. When blood testing is compared against skin prick testing, it shows a sensitivity and specificity of about 75% each, meaning it catches most true allergies but isn’t perfect. Skin prick testing is generally considered the more reliable first-line option.
Neither test alone is definitive. Results are interpreted alongside your actual symptom history. A positive test to an allergen you’ve never reacted to in real life doesn’t necessarily mean you need to avoid it.
The Numbers in 2024
Seasonal allergies are by far the most common type, affecting 25.2% of U.S. adults. Eczema affects 7.7%, and food allergies affect 6.7%. These categories overlap in many people, which is why the overall figure of 31.7% with any allergic condition is lower than the sum of the individual numbers. Allergies are not rare, not imaginary, and not something you simply outgrow in every case. Some childhood allergies (particularly to milk and egg) do resolve, but others, especially to pollen and shellfish, tend to persist or even appear for the first time in adulthood.