During a severe allergic reaction, epinephrine simply isn't enough.  MAST cells degranulate and release histimines.  Histamines recruit additional cells from the humoral immune system, causing fluid to move out of the blood and cells into the “third space”.  This causes swelling and sudden hypovolemia, which in turn leads to hypotension and shock. Diphenhydramine prevents this cascade of events from occurring by blocking the histamines.
 
Diphenhydramine must be administered as an intramuscular injection (commonly into the deltoid muscle),  due to the amount of time it will take for enteral medication to pass through the liver before entering systemic circulation.