Anaphylaxis is a medical emergency which involves an acute systemic (affecting the entire body) allergic reaction. It occurs following exposure to an antigen ( allergen ), to which a person was previously sensitized. Anaphylaxis can be caused by any allergen . However, the most common antigens to cause such a reaction are drugs , insect stings , certain food s and allergen immunotherapy injections.

An anaphylactic reaction starts when the allergen enters the bloodstream and reacts with an IgE class antibody. This causes cells to release histamine , which in turn causes the airways to constrict (causing difficulting in breathing), blood vessels to relax (lowering blood pressure) and the walls of the blood vessels to leak fluid (resulting in hives and swelling). The person may also go into shock . Anaphylaxis occurs immediately or at most within 2 hours of exposure to the allergen. Treatment is an epinephrine injection, which usually stops the reaction. Those who are allergic to bee stings or certain foods should carry a dose of epinephrine with them at all times.