What is the worst thing that could happen due to an allergic reaction?

Anaphylaxis. The most severe reaction that can occur after repeated exposure to something you are allergic to is called anaphylaxis. This rare but sudden condition causes itchy welts (hives), followed by wheezing, shortness of breath, and occasionally, swelling of the tongue or throat. Without treatment, swelling of the tongue or throat can sometimes worsen and block the airway.
Death. The worst thing that happen from an allergic reaction is death. This most serious allergic reaction is called anaphylaxis and can occur after exposure to certain foods, bees sting reaction, medication reactions or other unusual causes.
Death. Which is usually secondary to hypotension or asphyxiation. Less commonly hypoxemia from severe bronchospasm can also occur.
Death. Your lips, tongue and airways (large and small) could all swell shut. At the same time your blood pressure could drop, this is called anaphylaxis.
DEATH. Symptoms from an allergic reaction can range from an annoying symptoms such as itch or irritation to death.
Anaphylaxis. The worse allergic reaction which typically occurs to severe food allergy is anaphylaxis is defined as "a serious allergic reaction that is rapid in onset and may cause death". Symptoms may include hives or swollen lips, difficulty breathing and reduced blood pressure leading to shock.
Unfortunately, death. In rare cases, allergic reactions can be fatal. This may happen if the tongue and/or throat swell up and block off one's breathing. Death may also occur if the allergic reaction drops the blood pressure too low for too long, destroys the majority of one's blood cells or platelets, or severely damages the skin all over the body (mimicking a large burn).
Anaphylaxis. A true, severe allergic reaction can be life threatening. An allergen can trigger what is called an anaphylactic reaction, causing rash, wheezing, low blood pressure and other serious symptoms. Most "allergies" however, are much less severe. If you have concerns about possible allergies in your child, ask your pediatrician.
Anaphylaxis. Anaphylaxis is very dangerous, because the airway can swell up and close off. Anyone with a history of a severe food allergy (mouth or lip swelling) should have two epinephine ("epi") pens with him/her at all times. There are also blood tests that can help predict which foods or exposures could trigger a severe allergic reaction.