An acute hypersensitivity reaction that occurs from exposure to an allergen. It results from the release of histamine and histamine-like substances from mast cells, and can present with breathing difficulty due to narrowed airways, dizziness and hypotension, skin rash, weak pulse, nausea and vomiting. The disease is MONDO_0100053 (anaphylaxis). Also known as: anaphylactic shock, systemic anaphylaxis.