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.
If the allergen is something you breathe in from the air, your reaction will most likely affect your eyes, nose and lungs. If it's something you eat, it may affect your mouth, stomach and intestines. Food allergies also can cause skin rashes or even asthma symptoms.Allergy symptoms will vary depending on the type of allergen. To learn more, visit the allergy page that best applies to you: