Allergies occur when an overactive immune system produces antibodies (IgE) against substances in the environment (allergens) that are usually harmless. However, exposure to an allergen can result in symptoms that vary from a mild to life-threatening sudden reaction.
Anaphylaxis is a severe and sudden allergic reaction. It can occur when a susceptible person is exposed to an allergen (such as certain foods or an insect sting). Reactions usually begin within minutes of exposure and can progress rapidly over a period of up to two hours or more. Anaphylaxis is potentially life-threatening and always requires an emergency response.