A condition in which the body produces an abnormal immune response to certain antigens (called allergens), which include dust, pollen, certain foods and drugs, or fur. In allergic individuals these substances, which in a normal person would be destroyed by antibodies, react with pre-existing antibodies or trigger responses in primed T cells. A common mechanism is binding of allergen to IgE on mast cells. This causes the latter to secrete histamine and other vasoactive agents, leading to inflammation and other characteristic symptoms of the allergy (e.g. asthma or hay fever). This response is a type of hypersensitivity. See also anaphylaxis.

https://medlineplus.gov/ency/article/000812.htm Overview of common allergies, tests, and treatments, with illustrations of some typical symptoms