Final answer:
An allergen is a type of antigen that causes allergic reactions. Common allergens include pollen, mold, dust mites, certain foods, and insect stings.
Step-by-step explanation:
An allergen is a type of antigen that causes an allergic reaction. Allergens can be found in various substances, such as pollen, dust mites, mold, certain foods, insect stings, and medications. When an allergen enters the body, the immune system responds by producing an inflammatory response, leading to symptoms like sneezing, itching, and watery eyes.