Severe allergic reactions are caused by the extreme sensitivity of a body’s immune system to substances that are not normally dangerous. Common causes of severe allergic reactions include insect stings, peanuts, latex, and penicillin. Anaphylaxis is the medical term for a sudden, severe life-threatening allergic reaction. When a severe allergic reaction occurs, a person can rapidly develop a life-threatening constriction of the airway due to swelling.