Allergen

An allergen is a typically harmless substance that can provoke an allergic reaction in people who are sensitive to it, by activating their immune system. More precisely, it is an antigen that triggers a type-I hypersensitivity reaction in atopic individuals, mediated by immunoglobulin E (IgE) antibodies. While most humans produce significant IgE responses only to fight parasitic infections, some people generate inappropriate IgE against common environmental substances. Sensitivities differ greatly between individuals and even between animals, and the range of possible allergens is extremely broad.

The World Health Organization and International Union of Immunological Societies (WHO/IUIS) Allergen Nomenclature Sub-Committee maintains a public list of every protein—and some carbohydrates—that has been confirmed to trigger an IgE response in humans. Allergens come from many sources, including dust mite waste, pollen, and foods.

Examples

Food allergies are less common than food sensitivities, but certain foods—such as peanuts (a legume), tree nuts, seafood, and shellfish—cause serious reactions in many people. The United States Food and Drug Administration recognizes nine major food allergens, including organic fats and sulfites (chemicals often used as flavors or colors) at levels of 10 parts per million or more. Other countries have different official allergen lists due to genetic differences and varying exposure levels; Canada lists all nine US allergens plus mustard, while the European Union adds other organic grains.

An allergic reaction can occur through direct contact with an allergen: ingesting food or drink, inhaling pollen, perfume, or pet dander, or touching an allergy-causing plant. Other common triggers for serious allergies include wasp, fire ant, and bee stings, penicillin, and latex. A severe, life-threatening reaction is called anaphylaxis, which can be treated with sterile epinephrine to suppress the body’s overreaction and allow transport to a medical facility. Although allergic reactions usually require prior sensitization to a specific allergen, symptoms can sometimes appear on first exposure due to IgE cross-reactivity—where the immune system recognizes structurally similar proteins from a previous sensitization.

Non-IgE

Substances causing non-IgE-mediated, non-type-I hypersensitivity reactions are also broadly called allergens. A classic example is urushiol, a resin from poison ivy and poison oak that causes urushiol-induced contact dermatitis (a type IV hypersensitivity reaction) by altering skin cells so the immune system no longer recognizes them as part of the body. All contact dermatitides are type IV reactions, even when called “allergic contact dermatitis.” Various trees and wood products—such as paper, cardboard, and MDF—can also cause mild to severe allergy symptoms through touch or inhalation of sawdust, including asthma and skin rash.

Common allergens include animal products (cat dander, fur, cockroach calyx, wool, dust mite waste), drugs (penicillin, sulfonamides, salicylates), foods (celery, corn, eggs, fruit, pumpkin, eggplant, legumes like beans, peas, peanuts, and soybeans, milk, seafood, sesame, tree nuts like pecans and almonds, wheat), insect stings (bee and wasp venom, mosquito bites), mold spores, metals (nickel, chromium), latex, and wood. The top five allergens found in patch tests from 2005–06 were nickel sulfate (19.0%), balsam of Peru (11.9%), fragrance mix I (11.5%), quaternium-15 (10.3%), and neomycin (10.0%). Plant pollens causing hay fever include grasses (ryegrass, timothy-grass), weeds (ragweed, plantago, nettle, mugwort, lamb’s quarters, sorrel), and trees (birch, alder, hazel, hornbeam, horse chestnut, willow, poplar, plane, lime, olive, Ashe juniper, Alstonia scholaris).

Seasonal

Seasonal allergy symptoms typically occur during spring, summer, or fall, depending on when specific trees or grasses pollinate. For example, oak, elm, and maple pollinate in spring, while Bermuda, timothy, and orchard grasses pollinate in summer. Grass allergy is often linked to hay fever, with similar symptoms: rhinitis (sneezing, runny nose), allergic conjunctivitis (watery, itchy eyes), and an initial tickle on the roof of the mouth or back of the throat. Symptoms may be more severe depending on the season.