Anaphylaxis (allergic shock or anaphylactic shock) is the most severe form of an allergic reaction and can be life-threatening. It is a generalised allergic reaction, which often involves more than one body system (e.g., skin, respiratory, gastro-intestinal and cardiovascular).
It can be fatal within minutes, either through swelling that shuts off airways or through a dramatic drop in blood pressure. It must be treated as a medical emergency, requiring urgent medical attention.
Some individuals have a reaction, and the symptoms go away, only to return 2 to 3 hours later. This is called a "biphasic reaction." Biphasic anaphylactic reactions develop in as many as 20% of anaphylactic reactions. The second reaction can be less severe, equally severe, or more severe than the initial reaction, ranging in degree from mild symptoms to fatal reactions.