Allergies are among the most common health conditions in the United States, affecting nearly one in three adults. In 2024, 25.2% of U.S. adults had a seasonal allergy, 7.7% had eczema, and 6.7% had a food allergy. Allergies fall into several major categories: food, environmental, drug, and contact allergies, each with distinct triggers and symptoms.
How Allergic Reactions Work
Every allergic reaction starts with sensitization. Your immune system encounters a normally harmless substance, like pollen or peanut protein, and mistakenly flags it as dangerous. In people with a genetic tendency toward allergies, the immune system produces a specific type of antibody in response to that substance. These antibodies attach to immune cells throughout your body and wait.
The next time you encounter the same substance, those primed immune cells recognize it and release a flood of histamine and other inflammatory chemicals. Histamine is what causes the familiar symptoms: itching, swelling, sneezing, hives, and in severe cases, a dangerous drop in blood pressure. This whole process can unfold in minutes, which is why allergic reactions often feel sudden even though the underlying sensitization happened weeks, months, or years earlier.
Food Allergies
Food allergies affect roughly 8% of children and 6% of adults in the United States. Federal law now recognizes nine major food allergens, sometimes called the “Big 9”: milk, eggs, peanuts, tree nuts (almonds, walnuts, pecans), wheat, soybeans, fish, shellfish (crab, lobster, shrimp), and sesame. Sesame was added as the ninth allergen under the FASTER Act, and as of January 1, 2023, packaged foods containing sesame must declare it on the label.
Food allergy prevalence varies by demographic. Black non-Hispanic adults have the highest rates of diagnosed food allergies at 9.9%, compared with 6.4% in white non-Hispanic adults, 5.5% in Asian non-Hispanic adults, and 5.4% in Hispanic adults. Food allergies also tend to decrease with age, dropping from 7.4% in adults 18 to 44 down to 4.7% in those 75 and older.
Reactions range from mild (hives, tingling in the mouth, stomach cramps) to severe. The most dangerous food allergy reaction is anaphylaxis, a whole-body response that can cause throat swelling, difficulty breathing, and a sudden drop in blood pressure. People at risk for anaphylaxis typically carry an epinephrine auto-injector, which delivers a pre-measured dose to counteract the reaction while they get emergency help.
Early Introduction in Infants
Current guidelines from the FDA and the Dietary Guidelines for Americans recommend that infants at high risk of peanut allergy (those with severe eczema, egg allergy, or both) be introduced to age-appropriate peanut-containing foods as early as 4 to 6 months. This early exposure can reduce the risk of developing a peanut allergy. For high-risk babies, a skin prick test or blood test may be recommended first to determine the safest way to introduce peanut.
Environmental and Seasonal Allergies
Seasonal allergies are the most common type overall, affecting one in four U.S. adults. Women are significantly more likely to have them (29.5%) than men (20.7%). These allergies peak in adults between 45 and 64 years old, when prevalence reaches 27.7%, then gradually decline in older age groups.
The main environmental triggers include:
- Pollen from trees, grasses, and weeds. Pollen counts are typically highest in the morning and rise further on warm, windy days. Tree pollen dominates in spring, grass pollen in summer, and weed pollen (especially ragweed) in fall.
- Dust mites, microscopic creatures that live in bedding, mattresses, carpets, curtains, and upholstered furniture. They thrive in hot, humid environments and exist on every continent except Antarctica.
- Mold spores, which peak during hot, humid weather. Indoors, mold grows in damp areas with poor airflow like basements, kitchens, and bathrooms. Outdoors, it collects in leaf piles, mulch, hay, and grass.
- Pet dander and saliva. The actual allergen isn’t fur itself but tiny skin flakes and proteins secreted through your pet’s sweat glands and saliva. These proteins collect in skin, fur, and feathers.
- Cockroach debris. Proteins found in cockroach droppings, saliva, eggs, and dead body parts trigger allergic reactions, particularly in urban environments.
People living in nonmetropolitan (rural) areas are slightly more likely to have seasonal allergies (28.1%) than those in metropolitan areas (24.8%), possibly due to higher pollen exposure.
Drug Allergies
Penicillin is the most commonly reported drug allergy. About 10% of U.S. patients have a penicillin allergy noted in their medical record, but fewer than 1% are truly allergic when formally tested. The antibodies responsible for penicillin allergy often decrease over time, meaning many people who reacted to penicillin as children can tolerate it later in life.
This matters because a penicillin allergy label can limit your treatment options. If you’ve been told you’re allergic to penicillin but the reaction happened years ago, allergy testing can determine whether you’ve outgrown it. Other commonly reported drug allergens include sulfa antibiotics, aspirin, and nonsteroidal anti-inflammatory drugs.
Skin and Contact Allergies
Contact allergies develop when your skin touches a substance that triggers a localized immune response, producing an itchy, red rash called allergic contact dermatitis. Unlike immediate allergic reactions, contact dermatitis usually appears 12 to 72 hours after exposure, which can make it tricky to identify the trigger.
Nickel is one of the most common contact allergens. It’s found in jewelry, belt buckles, zippers, eyeglass frames, and many everyday metal objects. Balsam of Peru, a fragrance compound used in perfumes, toothpastes, mouth rinses, and flavorings, is another frequent cause. Latex, found in rubber gloves and some medical devices, can cause both contact dermatitis and more immediate allergic reactions in sensitized individuals.
Eczema, a chronic skin condition closely linked to allergies, affects 7.7% of U.S. adults. It’s more common in women (9.5%) than men (5.7%) and more prevalent in younger adults, with 9.1% of those 18 to 44 affected compared to 5.6% of those 75 and older.
How Allergies Are Diagnosed
The skin prick test is the most widely used diagnostic tool. A small amount of a suspected allergen is placed on your skin, usually on your forearm or back, and the skin is lightly pricked so the substance enters just below the surface. If you’re allergic, a small raised bump appears within about 15 minutes. A single session can test for up to 50 different substances, including pollen, mold, pet dander, dust mites, and foods.
Skin prick tests are generally reliable for airborne allergens. For food allergies, results can be less clear-cut, and additional testing or a supervised food challenge may be needed. People with severe eczema or psoriasis covering large areas of their arms and back may not have enough clear skin for testing. In those cases, blood tests that measure allergen-specific antibodies offer an alternative. Blood tests aren’t used for penicillin allergy, however.
Who Gets Allergies
Genetics play the largest role. The tendency to produce the antibodies that drive allergic reactions runs in families. If one or both of your parents have allergies, your risk is significantly higher, though you won’t necessarily be allergic to the same things they are.
Women are consistently more likely than men to have allergies across every category: seasonal allergies, eczema, and food allergies all show a clear sex-based difference. Where you live also plays a role. Adults in metropolitan areas have higher rates of eczema (7.9% vs. 6.4%) and food allergies (6.8% vs. 5.9%) compared to those in rural areas, while rural residents have slightly more seasonal allergies.