You get allergies because your immune system misidentifies a harmless substance, like pollen or a food protein, as a genuine threat and mounts a defensive response against it. This isn’t a sign that something is “wrong” with you exactly. It’s your immune system being overzealous, treating tree pollen or cat dander the way it would treat a parasite or pathogen. About 50 million Americans deal with some type of allergy each year, and several converging factors explain why you’re one of them.
What Happens Inside Your Body
An allergic reaction is a two-step process. The first time you encounter an allergen, your immune system quietly takes note. It produces a specific type of antibody called IgE, which attaches to mast cells and basophils (immune cells found throughout your skin, airways, and gut). You feel nothing during this “sensitization” phase. You’re essentially being primed.
The second time you encounter that same allergen, those armed immune cells recognize it and release a flood of chemical mediators. Histamine is the most well-known: it’s responsible for the itchy eyes, runny nose, sneezing, and hives that define a typical allergic reaction. But your body also releases other compounds. Leukotrienes contribute to inflammation and cause your airways to constrict, which is why allergies and asthma are so tightly linked. Prostaglandins add to the swelling in surrounding tissues. All of these chemicals working together produce the miserable constellation of symptoms you recognize as “allergies.”
Why Your Immune System Makes This Mistake
Your genetics play a major role. If one or both of your parents have allergies, asthma, or eczema, you’re significantly more likely to develop allergic conditions yourself. What you inherit isn’t a specific allergy to cats or ragweed but rather a general tendency for your immune system to overproduce IgE antibodies in response to common environmental proteins.
Beyond genetics, how your immune system was trained in early life matters enormously. The “Old Friends” hypothesis (an evolution of the older “hygiene hypothesis”) argues that humans co-evolved with certain environmental microbes and commensal bacteria that helped teach our immune systems to tolerate harmless substances. In modern, highly sanitized environments, children encounter fewer of these microbial teachers. Without that early education, the immune system is more likely to overreact to things like dust mites, pollen, or peanut proteins. This helps explain why allergies are far more prevalent in industrialized nations and why children raised on farms or with pets tend to have lower rates of allergic disease.
What You’re Reacting To
Environmental allergens include pollen from trees, grasses, and weeds, along with dust mites, mold spores, pet dander, and cockroach droppings. These are the triggers behind seasonal and year-round nasal allergies. Pollen allergies tend to follow a seasonal pattern: tree pollen dominates in spring, grass pollen in late spring and summer, and ragweed in fall.
Food allergies involve a different set of triggers. The nine major food allergens recognized in the United States are milk, eggs, peanuts, tree nuts, fish, shellfish, wheat, soybeans, and sesame. These account for the vast majority of serious food allergy reactions.
There’s also an interesting overlap between pollen and food allergies that catches many people off guard. If you’re allergic to birch tree pollen, you may notice itching or tingling in your mouth when you eat raw apples, cherries, carrots, or hazelnuts. Grass pollen allergies can cross-react with peaches, tomatoes, celery, and melons. Ragweed allergies sometimes trigger reactions to bananas, cucumbers, and zucchini. This is called oral allergy syndrome, and it happens because proteins in these foods are structurally similar to the pollen proteins your immune system already targets. Cooking the food typically breaks down these proteins enough to prevent a reaction.
Why Allergies Can Start at Any Age
Many people assume allergies are something you either have from childhood or don’t have at all. That’s not how it works. Your immune system can change its mind. Every time you’re exposed to a potential allergen, your immune system has an opportunity to develop a reaction to it. Several factors make adult-onset allergies more likely:
- Moving to a new region exposes you to an unfamiliar ecosystem of plants, molds, and insects your immune system hasn’t encountered before.
- Hormonal shifts during puberty, pregnancy, or menopause can change how your immune system behaves around allergens.
- Chronic stress weakens immune regulation, so something that never bothered you before can suddenly push you over the threshold.
- Repeated exposure is often the simple explanation. Insect stings are a classic example: most people tolerate their first sting fine, but later stings can trigger increasingly serious reactions as the immune system becomes sensitized.
People with a history of eczema or asthma are more prone to developing new allergies later in life, even if they’ve been symptom-free for years. Having one allergic condition signals that your immune system has the underlying tendency to overreact.
The Allergic March in Children
In children, allergic diseases often follow a characteristic progression. Eczema (atopic dermatitis) typically appears first, often in infancy. Food allergies may follow. Then, as the child grows, asthma and nasal allergies (allergic rhinitis) can develop in sequence. This pattern is known as the atopic march, with eczema serving as the “entry point” for subsequent allergic conditions.
The march isn’t inevitable, though. Roughly half of children with eczema never go on to develop asthma or allergic rhinitis. And sometimes the sequence is reversed, with respiratory allergies appearing before skin symptoms. But the pattern is common enough that pediatricians watch for it, because early eczema is one of the strongest predictors of future allergic disease.
Why Your Allergies May Be Getting Worse
If your seasonal allergies feel more intense than they did a decade ago, you’re not imagining it. Pollen seasons across North America have shifted dramatically. Between 1990 and 2018, pollen seasons started roughly 20 days earlier and lasted about 8 days longer. Total pollen concentrations increased by approximately 21% over the same period. A study published in the Proceedings of the National Academy of Sciences found that human-caused climate change was responsible for an estimated 50% or more of the shift in pollen season timing during recent years.
Warmer temperatures extend the growing season for allergenic plants and increase the amount of pollen each plant produces. Rising carbon dioxide levels further boost pollen output. The result is that even if your personal sensitivity hasn’t changed, the amount of allergen you’re exposed to each year is measurably higher than it used to be. For people right at the threshold of reacting, this increased pollen load can be what tips them from “mildly stuffy” into full-blown allergy symptoms.
Managing the Underlying Problem
Antihistamines are the most common first-line treatment because they block histamine, the chemical responsible for the bulk of your symptoms: sneezing, itching, runny nose, and watery eyes. They’re effective but only address part of the picture, since they don’t block leukotrienes or other inflammatory mediators.
Nasal corticosteroid sprays reduce the broader inflammation in your nasal passages and are generally more effective than antihistamines alone for persistent nasal symptoms. For people with specific, well-identified triggers, allergen immunotherapy (allergy shots or sublingual tablets) works by gradually retraining your immune system to tolerate the allergen rather than attack it. This is the closest thing to addressing the root cause rather than just managing symptoms, and it typically requires three to five years of consistent treatment to produce lasting results.
Practical avoidance still matters. Keeping windows closed during peak pollen hours (typically morning), using HEPA filters, showering after time outdoors, and encasing mattresses and pillows in dust-mite-proof covers can meaningfully reduce your daily allergen exposure and the intensity of your symptoms.