An allergic reaction is an immune system response to a specific substance to which a person is exposed. Allergic reactions are fairly common.
Most reactions happen soon after contact with an allergen.
An allergen is a trigger that causes the reaction after touching a certain part of the body. The blood may be exposed from an injection The blood or gut may be exposed from swallowing an allergen Usually these reactions are mild and can be treated at home with simple methods. However, some people have a sudden, life-threatening allergic reaction within minutes, called anaphylaxis . Anaphylaxis can progress rapidly and result in shock and even death if medical help is not obtained quickly. Usually, the first exposure produces only a very mild reaction or no reaction at all.