Anaphylaxis is a severe and potentially fatal allergic reaction. It may start suddenly within seconds or minutes, or take a few hours to develop following contact with an allergen which is a substance that is capable of producing an allergic reaction.
A severe anaphylactic reaction is sometimes known as anaphylactic shock. Anaphylaxis can cause the blood pressure to drop quickly, resulting in fainting or even sustained loss of consciousness. It can also cause severe breathing difficulties.