Tip - Allergy Symptoms

Types of Allergy Symptoms Allergy symptoms occur when your immune system overreacts to an allergen—something that usually is harmless, such as plant pollen, dust mites, molds, insect stings or food. If you have an allergy, your immune system acts as if the allergen were dangerous, releasing a chemical called histamine that causes allergy symptoms. Learn about headaches, a common symptom of allergies.