Allergy

An allergy is a specific type of exaggerated immune response where the body mistakenly identifies a ordinarily harmless substance (allergens, like pollen, pet dander, or certain foods) as a threat and launches a defense against it.^{[11][12][13][14]}

Allergic diseases are the conditions that arise as a result of allergic reactions, such as hay fever, allergic conjunctivitis, allergic asthma, atopic dermatitis, food allergies, and anaphylaxis.^[1] Symptoms of the above diseases may include red eyes, an itchy rash, sneezing, coughing, a runny nose, shortness of breath, or swelling.^[15] Note that food intolerances and food poisoning are separate conditions.^[3][4]

Common allergens include pollen and certain foods.^[11] Metals and other substances may also cause such problems.^[11] Food, insect stings, and medications are common causes of severe reactions.^[2] Their development is due to both genetic and environmental factors.^[2] 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.^[16] Diagnosis is typically based on a person's medical history.^[3] Further testing of the skin or blood may be useful in certain cases.^[3] Positive tests, however, may not necessarily mean there is a significant allergy to the substance in question.^[17]

Early exposure of children to potential allergens may be protective.^[5] Treatments for allergies include avoidance of known allergens and the use of medications such as steroids and antihistamines.^[6] In severe reactions, injectable adrenaline (epinephrine) is recommended.^[7] Allergen immunotherapy, which gradually exposes people to larger and larger amounts of allergen, is useful for some types of allergies such as hay fever and reactions to insect bites.^[6] Its use in food allergies is unclear.^[6]

Allergies are common.^[10] In the developed world, about 20% of people are affected by allergic rhinitis,^[18] food allergy affects 10% of adults and 8% of children,^[19] and about 20% have or have had atopic dermatitis at some point in time.^[20] Depending on the country, about 1–18% of people have asthma.^[21][22] Anaphylaxis occurs in between 0.05–2% of people.^[23] Rates of many allergic diseases appear to be increasing.^[7][24][25] The word "allergy" was first used by Clemens von Pirquet in 1906.^[2]