Allergies, also known as allergic diseases, refer a number of conditions caused by the hypersensitivity of the immune system to typically harmless substances in the environment. These diseases include hay fever, food allergies, atopic dermatitis, allergic asthma, and anaphylaxis. Symptoms may include red eyes, an itchy rash, sneezing, coughing, a runny nose, shortness of breath, or swelling. Note: food intolerances and food poisoning are separate conditions.

Allergy
Hives are a common allergic symptom.
Specialty	Immunology
Symptoms	Red eyes, itchy rash, vomiting, runny nose, shortness of breath, swelling, sneezing, and cough.
Types	Hay fever, food allergies, atopic dermatitis, allergic asthma, anaphylaxis
Causes	Genetic and environmental factors
Diagnostic method	Based on symptoms, skin prick test, blood test
Differential diagnosis	Food intolerances, food poisoning
Prevention	Early exposure to potential allergens
Treatment	Avoiding known allergens, medications, allergen immunotherapy
Medication	Steroids, antihistamines, epinephrine, mast cell stabilizers, antileukotrienes
Frequency	Common

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

Early exposure of children to potential allergens may be protective. Treatments for allergies include avoidance of known allergens and the use of medications such as steroids and antihistamines. In severe reactions, injectable adrenalin (epinephrine) is recommended. 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. Its use in food allergies is unclear.

Allergies are common. In the developed world, about 20% of people are affected by allergic rhinitis, about 6% of people have at least one food allergy, and about 20% have or have had atopic dermatitis at some point in time. Depending on the country, about 1–18% of people have asthma. Anaphylaxis occurs in between 0.05–2% of people. Rates of many allergic diseases appear to be increasing. The word "allergy" was first used by Clemens von Pirquet in 1906.