Allergens cause your immune system to overreact to substances that are normally harmless, triggering symptoms that can affect your skin, airways, digestive system, and in severe cases, your entire body. About 31.7% of U.S. adults have at least one diagnosed allergic condition, making allergies one of the most common chronic health issues. What happens after exposure depends on the type of allergen, how it enters your body, and how sensitized your immune system has become.
How Allergens Trigger a Reaction
An allergic reaction is a case of mistaken identity. Your immune system encounters a protein in pollen, food, pet dander, or another substance and flags it as dangerous, even though it poses no real threat. The first time this happens, you won’t feel anything. Your body quietly produces a specific type of antibody (called IgE) designed to recognize that protein. These antibodies attach to immune cells called mast cells, which are stationed throughout your skin, airways, and gut. This silent preparation stage is called sensitization.
The next time you encounter the same substance, those antibodies recognize it immediately. The allergen latches onto the IgE antibodies sitting on your mast cells, and the cells essentially burst open, releasing a flood of chemicals. Histamine is the most well-known of these chemicals, and it’s responsible for many of the symptoms you feel: itching, swelling, mucus production, and the dilation of blood vessels. This cascade can begin within seconds to minutes of exposure.
One important distinction: having these antibodies doesn’t guarantee you’ll have symptoms. Research shows that over 36% of young children and nearly 45% of people over age six test positive for antibodies to at least one allergen, but many of them never develop actual allergic disease. Sensitization is a risk factor, not a diagnosis on its own.
Symptoms by Allergen Type
The symptoms an allergen causes depend largely on how it gets into your body.
Airborne allergens like pollen, dust mites, and pet dander primarily affect your nose and eyes. Seasonal allergies (the most common type, affecting about 25% of U.S. adults) cause sneezing, a runny or stuffy nose, watery and red eyes, itching of the nose and roof of the mouth, and fatigue.
Food allergens produce a wider range of effects. The nine most common food allergens in the U.S. are milk, eggs, fish, shellfish, tree nuts, peanuts, wheat, soybeans, and sesame. Reactions typically appear within minutes to two hours after eating and can include tingling in the mouth, swelling of the lips or throat, hives, stomach cramps, vomiting, diarrhea, and nasal congestion.
Insect stings often cause pain and significant swelling at the sting site, but can also produce body-wide hives, skin flushing and warmth, coughing, chest tightness, and shortness of breath.
Medication allergens can trigger hives, itchy skin or rash, facial swelling, wheezing, dizziness, and digestive symptoms like vomiting or diarrhea.
Contact allergens cause eczema, a skin condition affecting about 7.7% of adults. The skin itches, forms discolored patches (red or brown, depending on skin tone), and may flake, peel, or crack.
When Allergies Turn Dangerous
Anaphylaxis is the most severe form of allergic reaction, and it involves multiple organ systems at once. A person might develop hives alongside difficulty breathing, or throat swelling combined with a dangerous drop in blood pressure. No single symptom defines anaphylaxis. It’s the combination of skin involvement with respiratory distress, cardiovascular changes, or severe digestive symptoms that makes it life-threatening.
Even after symptoms resolve, roughly 4.7% of anaphylactic reactions come back in a second wave, known as a biphasic reaction. This is why hospitals typically observe patients for several hours after treating anaphylaxis. Peanut allergies carry a notably higher biphasic risk, with about 9.6% of peanut-triggered reactions recurring, while milk-triggered anaphylaxis rarely follows a biphasic pattern. People whose initial reaction involved multiple organs, or who have a history of chronic hives, face a higher chance of this rebound.
Cross-Reactivity Between Allergens
Sometimes an allergen causes reactions to seemingly unrelated substances. This happens because proteins in different foods can look structurally similar to proteins in pollen, confusing your immune system. Up to 50 to 75% of adults allergic to birch tree pollen experience tingling or itching in the mouth when they eat certain raw fruits, vegetables, or nuts. Apples, carrots, peanuts, almonds, and hazelnuts are common triggers for people with birch pollen allergies.
Grass pollen allergy can cross-react with peaches, celery, tomatoes, melons, and oranges. Ragweed allergy sometimes causes mouth symptoms when eating bananas, cucumbers, melons, or zucchini. This phenomenon, called oral allergy syndrome, is usually mild and limited to the mouth and throat. Cooking the food typically breaks down the problematic proteins enough to prevent symptoms.
How Allergies Are Identified
Skin prick testing is the most common method. Small drops of allergen extracts are placed on your skin (usually your forearm or back), and a tiny lancet scratches each drop into the surface. After about 15 minutes, your skin is checked for raised, itchy bumps called wheals. Larger bumps generally indicate greater sensitivity. The whole process takes 20 to 40 minutes.
Skin tests are reliable for airborne allergens like pollen, pet dander, and dust mites, and can help with food allergy diagnosis. But they aren’t perfect. False positives (reacting to something you’re not truly allergic to) and false negatives (no reaction despite a real allergy) both occur. You can even get different results on different days. Certain medications can also suppress the skin’s response. One asthma medication, omalizumab, can interfere with results for six months or longer after you stop taking it.
Blood tests offer an alternative by measuring the level of allergen-specific antibodies in your blood. Historically, antibody levels above 0.35 kUA/L were considered positive, though newer research suggests lower thresholds may be more accurate for certain allergens. For cat and dog allergies in young adults, levels as low as 0.12 and 0.20 kUA/L respectively provided the best prediction of actual symptoms.
Who Gets Allergies
Women are consistently more likely than men to have allergic conditions. In 2024, 29.5% of women had seasonal allergies compared to 20.7% of men. The same pattern holds for eczema (9.5% vs. 5.7%) and food allergies (8.3% vs. 5.1%).
Seasonal allergies peak in middle age. Adults between 45 and 64 have the highest rate at 27.7%, while those 75 and older drop to 21.7%. Food allergies trend the opposite direction, declining with age from 7.4% in adults 18 to 44 down to 4.7% in those 75 and older. Eczema also decreases steadily with age, from 9.1% in younger adults to 5.6% in those over 75.
Race and geography matter too. White adults have the highest rate of seasonal allergies (28.5%), while Black adults have the highest rate of food allergies (9.9%). People living in rural areas report more seasonal allergies than those in cities, but city dwellers have higher rates of eczema and food allergies.