It's an extreme response to an allergen.
A life-threatening allergic reaction (anaphylaxis) is a severe reaction to a specific allergen, or allergic trigger. Common triggers can include but are not limited to food, biting or stinging insects, medications and latex. Exercise-induced anaphylaxis is also possible, and sometimes anaphylaxis has no apparent cause at all (this is known as idiopathic anaphylaxis).
What happens inside the body when anaphylaxis occurs?
The first step occurs when an allergen enters the body and causes antibodies (special proteins made by the body’s immune system) to form. These antibodies sensitize certain immune cells (called mast cells and basophils) to react the next time they encounter that specific allergen. When the allergen and these sensitized immune cells meet again, the antibodies cause the release of certain molecules (including histamine) that produce the immediate symptoms of anaphylaxis.