Allergy is a condition in which the immune system reacts abnormally to a substance, known as an allergen, that is typically harmless to most people. Common allergens include pollen, dust mites, pet dander, certain foods, and insect stings. Symptoms can range from mild, such as sneezing and itching, to severe, including anaphylaxis, which is a life-threatening reaction. Allergies can be diagnosed through various tests and are often managed through avoidance, medications, or immunotherapy.