An allergen is a substance that can cause an allergic reaction in some people. Common allergens include pollen, dust mites, pet dander, certain foods, and insect stings. When a person with a sensitivity to an allergen comes into contact with it, their immune system may overreact, leading to symptoms such as sneezing, itching, or swelling. Allergens can be found in various environments, including homes, workplaces, and outdoor areas. Managing exposure to allergens often involves avoiding known triggers and may include treatments like antihistamines or allergy shots. Understanding allergens is essential for those with allergies to maintain their health and well-being.