An allergen is a substance that can cause an allergic reaction in some people. Common allergens include pollen, dust mites, pet dander, and certain foods like peanuts or shellfish. When a person with an allergy comes into contact with an allergen, their immune system mistakenly identifies it as harmful and triggers a response, which can lead to symptoms such as sneezing, itching, or swelling. Allergens can be found in various forms, including airborne particles, foods, and insect stings. The severity of allergic reactions can vary from mild to life-threatening, depending on the individual and the allergen involved. Understanding allergens is crucial for managing allergies and preventing reactions.