The most severe case of an allergic reaction is called anaphylactic allergic reactions. The are many items this can happen with, one of the most common is peanuts. Peanuts are very popular in the United States, it is in a lot of foods.
Its is an estimate that around 1.2% of the United States is allergic to peanuts. The actual allergy is caused just like any other allergic reaction. Your body assumes that the peanuts are harmful. So your body reacts to counter the peanuts with histamine, but doesnt just react it over reacts and we overdose on histamin causing the general symptoms of an allergic reaction.
You can see the general peanut allergy symptoms here. But if you are unsure, please make sure you contact your docto. Going in to anaphylactic shock can kill.