Anaphylaxis is defined as "a serious allergic reaction that is rapid in onset and may cause death". It typically results in a number of symptoms including an itchy rash, throat swelling, and low blood pressure. Common causes include insect bites, foods, and medications.

On a pathophysiologic level, anaphylaxis is caused by the release of mediators from certain types of white blood cells triggered either by immunologic or non-immunologic mechanisms. It is diagnosed based on the presenting symptoms and signs. The primary treatment is injection of epinephrine, with other measures being complementary.

Worldwide 0.05–2% of people are estimated to have anaphylaxis at some point in their life and rates appear to be increasing. The term comes from the Greek words ἀνά ana, against, and φύλαξις phylaxis, protection.

Anaphylaxis typically presents with many different symptoms over minutes or hours with an average onset of 5 to 30 minutes if exposure is intravenous and 2 hours for foods. The most common areas affected include: skin (80–90%), respiratory (70%), gastrointestinal (30–45%), heart and vasculature (10–45%), and central nervous system (10–15%) with usually two or more being involved.