MD 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. 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. For some people, repeated exposures may lead to more serious reactions.