Allergens are substances that are harmless to most people but can trigger an exaggerated immune response in sensitive individuals. They may come from food, the air, household dust, mold, animals, insects, medications or everyday personal-care and hygiene products [1].
It is important to understand that an allergen is not the same as a toxin or a "harmful substance." In allergy, the problem occurs because the immune system mistakenly identifies a specific substance as a threat and triggers a defensive response [1].
Food allergies can cause mild symptoms, but in some people they can also lead to serious reactions, including anaphylaxis. Food allergy should therefore not be treated as simply a "sensitive stomach" [2].
In the United States, the nine major food allergens subject to special labeling requirements are milk, eggs, fish, crustacean shellfish (such as shrimp, crab and lobster), tree nuts, peanuts, wheat, soy and sesame. Sesame became the ninth major allergen required to be labeled as of January 1, 2023 [3].
Milk allergy is an immune system reaction to milk proteins, most commonly casein and whey proteins. It is more common in children but can also occur in adults [2].
Milk allergy is not the same as lactose intolerance. In lactose intolerance, the problem is reduced ability to digest milk sugar, while allergy involves the immune system and may cause a more serious reaction [2].
Eggs are among the common allergens in childhood. Reactions may be triggered by proteins in the egg white, yolk or both, although egg-white proteins are more commonly associated with allergic reactions [2].
In some people, egg allergy becomes less severe over time, but this should not be tested independently at home if a serious reaction has occurred before. Reintroduction of a potential allergen should be done only with advice from an allergist [2].
Peanuts are among the best-known allergens because they can cause severe and rapid reactions in some people. Botanically, peanuts are legumes rather than tree nuts, but they are often discussed together with tree nuts because of their importance in food allergy and the risk of cross-contact [2].
Tree nuts include almonds, hazelnuts, walnuts, cashews, pistachios and other similar nuts. A person may be allergic to one tree nut but not necessarily to all of them, although assessment should always be individualized [2].
Fish and seafood allergies often persist into adulthood. Reactions most commonly occur after eating the food, but in highly sensitive people symptoms may sometimes occur after inhaling aerosols or steam during food preparation [2].
This broader group includes different types of fish, crustaceans such as shrimp, crab and lobster, and mollusks such as mussels, oysters and squid. It is important to distinguish this broader seafood category from the U.S. regulatory category "Crustacean shellfish," which is one of the nine major allergen categories for labeling [2], [3].
Wheat allergy is an immune reaction to wheat proteins. It is not the same as celiac disease, which is an autoimmune disease triggered by gluten, nor is it the same as non-celiac gluten sensitivity [2].
Soy is a common ingredient in processed foods, plant-based drinks, protein products, sauces and some ready-made meals. People with a confirmed soy allergy should read ingredient lists carefully [2].
Sesame is found in baked goods, bread, tahini, hummus, spreads, sauces, spice blends and some snacks. It can therefore be "hidden" in foods that a person may not immediately recognize as an obvious source of sesame [3].
Since 2023, sesame has been included in the U.S. list of major allergens that must be clearly labeled, together with milk, eggs, fish, crustacean shellfish, tree nuts, wheat, peanuts and soy [3].
The most common outdoor allergens include pollen from trees, grasses and weeds, as well as mold spores. They most often cause symptoms of allergic rhinitis such as sneezing, a runny nose, nasal congestion, itching of the nose and eyes, and watery eyes [4].
Pollen is especially important in seasonal allergies. Symptoms often appear in spring, summer or early fall depending on whether a person is sensitive to tree, grass or weed pollen [4].
Tree pollen is a more common trigger in spring, grass pollen during late spring and summer, while weed pollen often causes problems in late summer and fall. The exact timing depends on geographic location, climate and weather conditions [4].
Windy, dry days can worsen symptoms because pollen spreads more easily through the air. Rain may temporarily reduce the amount of pollen in the air, but moisture can promote mold growth, which may trigger other allergy symptoms in some people [4].
The most common indoor allergens include dust mites, mold, pet allergens from skin dander, saliva and urine, as well as cockroach particles and other components of household dust. These allergens can cause symptoms throughout the year [5].
If a person has persistent nasal congestion, morning sneezing, coughing, itchy eyes or worsening symptoms during cleaning or while spending time in the bedroom, indoor allergens may be possible triggers [5].
Dust mites are microscopic organisms most commonly found in bedding, mattresses, pillows, carpets, curtains and upholstered furniture. Their allergens can cause allergic rhinitis and worsen asthma in sensitive individuals [5].
Dust mites thrive in warm, humid environments, which makes the bedroom especially important for symptom control. Washing bedding, reducing dust, using protective mattress and pillow covers and controlling humidity may help people with confirmed dust mite sensitivity [5].
Mold can be present both outdoors and indoors. Indoors, it most commonly develops in damp rooms, bathrooms, basements, around windows, in damp walls or in areas with water leaks [5].
Mold allergy can cause sneezing, nasal congestion, a runny nose, coughing, eye irritation and worsening respiratory symptoms. In people with asthma, mold exposure can further worsen symptom control [5].
Pet allergy is usually not a reaction to hair alone. Allergens are found in skin dander, saliva and urine and can then spread easily to hair, furniture, clothing, carpets and bedding [5].
Cats and dogs are the most common examples, but other pets can also trigger allergic reactions. Symptoms may occur immediately after contact or later because allergens can remain in the environment for a long time [5].
In some environments, cockroach allergens can be important triggers of allergic rhinitis and asthma. Body parts, feces and insect debris can be present in household dust and trigger symptoms in sensitive individuals [5].
Household dust is not a single allergen but a mixture of different particles. It may contain dust mites, pet dander, mold spores, pollen, fibers, insect debris and other small particles [5].
Stings from bees, wasps, hornets and similar insects can cause a local reaction, but in allergic individuals they can also trigger a serious systemic reaction. A large local swelling is not always the same as anaphylaxis, but difficulty breathing, throat swelling, dizziness or a drop in blood pressure require emergency care [1].
People who have previously had a serious reaction to an insect sting should discuss a protection plan with a physician, including the possibility of carrying an epinephrine auto-injector and, when appropriate, allergen immunotherapy [1].
Medications can also cause allergic reactions. Antibiotics, especially penicillins, are commonly discussed, as are nonsteroidal anti-inflammatory drugs, contrast agents and some other medications [1].
It is important not to label a person as "allergic to a medication" without proper assessment, because an incorrect allergy label can later limit treatment options. On the other hand, if the reaction was serious, the medication should not be used again without professional evaluation [1].
Certain cosmetic ingredients, fragrances, preservatives, hair dyes, metals such as nickel and ingredients in hygiene products can cause contact dermatitis. This reaction most often appears on the skin as redness, itching, burning, flaking or rash [1].
Not every reaction to cosmetics is a true allergy. Sometimes it is irritation, especially when the skin barrier is damaged, products are used aggressively or frequently, or several active ingredients are combined [1].
The most useful approach is to look for a pattern in symptoms. If symptoms occur after a specific food, it is important to note what was eaten, how quickly symptoms appeared and what they were like. If symptoms occur during a certain season, after cleaning, around animals or in damp rooms, environmental allergens are more likely [1].
Allergy tests can help, but they should not be interpreted on their own. A positive test means that sensitization is present, but it does not always prove that the allergen actually causes symptoms. Results should always be considered together with the person's real-life reaction [2].
The most common food allergens include milk, eggs, peanuts, tree nuts, fish, crustacean shellfish, wheat, soy and sesame. Common environmental allergens include pollen, dust mites, mold, pet and cockroach allergens, while allergic reactions can also be triggered by insect stings, medications and certain contact allergens [1], [3], [5].
An allergy is not assessed only by identifying a possible trigger, but also by considering when symptoms occur, how quickly they appear, how severe they are and whether they recur with the same exposure. With mild symptoms, tracking triggers can be useful, but throat swelling, choking, dizziness, fainting or rapid worsening requires emergency medical attention [1].
[1] American Academy of Allergy, Asthma & Immunology (AAAAI). Allergies Symptoms, Diagnosis, Management & Treatment. Accessed 2026.
https://www.aaaai.org/conditions-treatments/allergies
[2] American Academy of Allergy, Asthma & Immunology (AAAAI). Food Allergy. Accessed 2026.
https://www.aaaai.org/tools-for-the-public/conditions-library/allergies/food-allergy-ttr
[3] U.S. Food and Drug Administration (FDA). Food Allergies. 2026.
https://www.fda.gov/food/nutrition-food-labeling-and-critical-foods/food-allergies
[4] American Academy of Allergy, Asthma & Immunology (AAAAI). Hay Fever | Rhinitis Symptoms, Diagnosis, Management & Treatment. Accessed 2026.
https://www.aaaai.org/conditions-treatments/allergies/hay-fever-rhinitis
[5] American Academy of Allergy, Asthma & Immunology (AAAAI). Indoor Allergens. Accessed 2026.
https://www.aaaai.org/tools-for-the-public/conditions-library/allergies/indoor-allergens-ttr
Health Disclaimer: This content is for educational and informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional before starting or changing any supplement, especially if you are pregnant, breastfeeding, have a medical condition, or take medication.
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