An allergy is a condition in which the immune system reacts unusually to a substance, known as an allergen. Common allergens include pollen, dust mites, pet dander, certain foods like peanuts and shellfish, and insect stings. When a person with an allergy comes into contact with an allergen, their body may produce symptoms such as sneezing, itching, hives, or even more severe reactions like anaphylaxis. Allergies can be diagnosed through skin tests or blood tests that identify specific sensitivities. Treatment options often include avoiding known allergens, taking medications like antihistamines, or receiving allergy shots, which help desensitize the immune system to the allergen over time.