Allergies, also known as allergic diseases, are a number of conditions caused by hypersensitivity of the immune system to something in the environment that usually causes little or no problem in most people.[1] These diseases include hay fever, food allergies, atopic dermatitis, allergic asthma, and anaphylaxis.[2] Symptoms may include red eyes, an itchy rash, runny nose, shortness of breath, or swelling.[3] Food intolerances and food poisoning are separate conditions.[4][5]
Common allergens include pollen and certain food. Metals and other substances may also cause problems.[1] Food, insect stings, and medications are common causes of severe reactions. Their development is due to both genetic and environmental factors.[6] The underlying mechanism involves immunoglobulin E antibodies (IgE), part of the body's immune system, binding to an allergen and then to a receptor on mast cells or basophils where it triggers the release of inflammatory chemicals such as histamine.[7] Diagnosis is typically based on a person's medical history. Further testing of the skin or blood may be useful in certain cases.[5] Positive tests, however, may not mean there is a significant allergy to the substance in question.[8]