Final answer:
An allergy is a disorder in which the immune system overreacts to a harmless antigen in the environment, causing symptoms like sneezing and itching.
Step-by-step explanation:
An allergy is a disorder in which the immune system makes an inflammatory response to a harmless antigen or allergen in the environment. Allergens can include pollen, dust mites, certain foods, insect stings, and medications. When a person with an allergy is exposed to these substances, their immune system overreacts and causes symptoms such as sneezing, itching, and swelling.