Hypersensitivity

Allergy is the exaggerated response of the immune system to certain environmental antigens that most people tolerate without incident; the term itself combines 'allo' (altered) and 'erg' (reaction). Substances capable of provoking such a response are called allergens — common examples include dust mites, pollens and certain proteins in insect venom. An allergic reaction typically begins within seconds of contact with the allergen and lasts roughly half an hour, producing symptoms such as sneezing, watery eyes, a runny nose and difficulty breathing; hay fever and asthma are familiar everyday examples of allergic disease. Mechanistically, allergy is a form of overactive immune response mediated by IgE antibodies and mast cells — IgE bound to mast cells triggers the release of chemicals such as histamine and serotonin, which drive the visible symptoms. …