Anaphylaxis is a total body allergic reaction, which can range from mild to severe, but can be life-threatening.
We think of anaphylaxis due to food allergy and exposure, or venom allergy and a sting; but it can also occur with medications, and less commonly with excessive exposure to environmental allergens.
No one understands why an allergic reaction proceeds quickly to involve more than one organ system, and why it remains unchecked by the body.
Happily, it can be treated by an Epinephrine injection. Generally antihistamines and corticosteroids are used to treat intial symptoms as well.