Allergies happen when your immune system treats a harmless substance, like pollen or peanut protein, as a genuine threat. Nearly one in three American adults has a diagnosed seasonal allergy, eczema, or food allergy, and that number has been climbing for decades. The causes range from your genetic makeup to the specific substances you encounter, and even the era you were born in plays a role.
How Your Immune System Creates an Allergy
An allergy is fundamentally a case of mistaken identity. The first time you encounter an allergen, your immune system may flag it as dangerous. Specialized immune cells release signaling molecules that instruct other cells to produce a type of antibody called IgE. These IgE antibodies attach themselves to mast cells, which are stationed throughout your skin, airways, and gut. At this point you feel nothing. Your body is simply “sensitized,” primed to react the next time it sees that substance.
When you encounter the same allergen again, it locks onto those waiting IgE antibodies. This triggers the mast cells to burst open and flood the surrounding tissue with histamine and other inflammatory chemicals. Histamine is what causes the familiar symptoms: itchy eyes, a runny nose, hives, swelling, or in severe cases, a dangerous drop in blood pressure called anaphylaxis. The whole process can unfold in minutes, which is why allergic reactions often feel sudden even though the groundwork was laid during a previous, symptom-free exposure.
Airborne Allergens
Pollen is the most widespread airborne trigger. Trees, grasses, and weeds all release microscopic spores that travel on the wind, and their peak times vary by season. Tree pollen typically dominates in spring, grass pollen in late spring and summer, and weed pollen in fall. Pollen levels tend to be highest in the early morning, roughly between 5 a.m. and 10 a.m., and spike further on warm, windy days.
Mold spores are another major culprit. They thrive in hot, humid conditions and can be found both outdoors (in leaf litter, compost, and soil) and indoors (in bathrooms, basements, and anywhere moisture accumulates). Unlike pollen, mold doesn’t follow a neat seasonal calendar. It can trigger symptoms year-round in the right environment.
Dust mites and pet dander cause perennial allergies, meaning they persist regardless of season. Dust mites are microscopic creatures that feed on dead skin cells and flourish in warm, humid environments like bedding, upholstered furniture, and carpeting. Pet dander, the tiny flakes of skin shed by cats, dogs, and other furry animals, becomes airborne and clings to fabrics. These triggers are especially persistent indoors, where concentrations can build up over time.
Food Allergens
Food allergies affect roughly 8% of children and 6% of adults in the United States, and the nine foods responsible for the vast majority of reactions are milk, eggs, fish, shellfish, tree nuts, peanuts, wheat, soybeans, and sesame. These are sometimes called the “Big 9,” and U.S. food labeling laws require manufacturers to declare them on packaging.
Children often outgrow allergies to milk, eggs, wheat, and soy. Allergies to peanuts, tree nuts, fish, and shellfish are more likely to persist into adulthood. Food allergies can also appear for the first time in adults, particularly to shellfish, which many people eat only occasionally. Lower frequency of exposure to a food may actually make it more likely to emerge as a new allergy later in life.
Contact Allergens
Some allergic reactions require direct skin contact. The most common contact allergen is nickel, a metal found in jewelry, belt buckles, zippers, and phone cases. Fragrance mixtures rank high as well, showing up in perfumes, lotions, soaps, and household cleaners. Balsam of Peru, a resin used as a flavoring and fragrance ingredient, is another frequent offender. Even certain antibiotic ointments applied to the skin can cause a contact allergy in sensitized individuals.
Contact allergies produce a delayed reaction. Instead of the minutes-long response seen with airborne or food allergens, a red, itchy, blistering rash typically develops 12 to 72 hours after exposure. This delay often makes it harder to identify the trigger, since the rash doesn’t appear until long after the contact occurred.
Insect Stings and Medications
Venom from stinging insects, particularly honeybees, wasps, and certain ant species, is a common cause of severe allergic reactions. Unlike bees, which can sting only once, wasps can sting multiple times, potentially delivering a larger dose of venom. For most people a sting causes localized pain and swelling, but in someone who has been sensitized, it can trigger full-body anaphylaxis within minutes.
Medications can also cause allergic reactions. Antibiotics (especially penicillin-type drugs) and anti-inflammatory pain relievers are among the most frequent triggers, but virtually any medication has the potential to cause a reaction in a susceptible person. Drug allergies can develop after years of using the same medication without problems, which catches many people off guard.
Genetics and Family History
Your likelihood of developing allergies is heavily influenced by your genes. If one parent has allergies, you have a meaningfully higher chance of developing them yourself, and the risk climbs further if both parents are affected. What you inherit isn’t necessarily the same specific allergy your parents have. Rather, you inherit a tendency for your immune system to overproduce IgE antibodies in response to otherwise harmless substances. This general predisposition is called atopy, and it underlies allergic rhinitis, eczema, asthma, and food allergies alike.
That said, genetics alone doesn’t explain the picture. Whether those inherited tendencies actually produce symptoms depends on the specific environmental exposures you encounter throughout your life, which is why allergies can be somewhat unpredictable even within the same family.
Early Childhood Environment
One of the more compelling explanations for rising allergy rates is the hygiene hypothesis: the idea that modern, cleaner living conditions deprive the developing immune system of the microbial exposure it needs to calibrate properly. Without enough early encounters with harmless bacteria and other microbes, the immune system is more likely to overreact to benign substances like pollen or food proteins.
The evidence supporting this idea comes from several directions. Households with dogs have richer, more diverse microbial environments, and in animal studies, exposure to dust from those homes prevented the development of allergy-related asthma. A Swedish study found that families who washed dishes by hand, a less thorough method of eliminating microbes than machine dishwashing, had children with lower rates of eczema. Children whose parents cleaned dropped pacifiers by putting them in their own mouths first were less likely to develop allergic sensitization, eczema, or asthma by 18 months of age. Children born vaginally or who have older siblings tend to have greater microbial diversity in their gut during infancy, which is associated with lower allergy risk.
None of this means that cleanliness is bad. It means the immune system develops in conversation with the microbial world, and when that conversation is too limited in early life, the system is more prone to misfiring.
Climate Change and Longer Pollen Seasons
If your seasonal allergies feel worse than they did a decade ago, they probably are. A study published in the Proceedings of the National Academy of Sciences found that pollen seasons across North America lengthened by about 20 days and pollen concentrations increased by 21% between 1990 and 2018. Both trends are strongly linked to rising temperatures. Human-caused climate change accounted for roughly half of the lengthening of pollen seasons and about 8% of the increase in pollen concentrations.
Warmer temperatures cause plants to start releasing pollen earlier in the year and to continue later into fall. Higher carbon dioxide levels also stimulate plants to produce more pollen per plant. These shifts mean that people with seasonal allergies are exposed to more pollen over a longer window, which can worsen existing allergies and potentially sensitize people who previously had no problems.
Why Allergies Can Start in Adulthood
Many people assume allergies are something you either have as a child or don’t have at all. In reality, new allergies can develop at any age. Moving to a new geographic area exposes you to plant species and mold varieties your immune system hasn’t encountered before. Changes in your living situation, like getting a pet for the first time, introduce new allergens. Hormonal shifts, illness, and changes in immune function can also alter how your body responds to substances it previously tolerated.
The precise reason one person develops a new allergy at 40 while another never does remains partly mysterious. It likely comes down to a complex interplay between inherited susceptibility and the timing, dose, and frequency of allergen exposure. Infrequent contact with a potential allergen, like eating shellfish only a few times a year, may actually raise the odds of sensitization compared to regular exposure.