Anaphylaxis (say "ann-uh-fuh-LAK-suss") is a severe allergic reaction that affects the entire body (systemic). It can occur within a few seconds or minutes after a person is exposed to a substance (allergen or antigen).

Symptoms and signs of a severe allergic reaction may include:

• Itching.
• Raised, red bumps on the skin (hives or wheals).
• Wheezing or difficulty breathing.
• Rapid swelling, either in one area or over the entire body. Swelling is most serious when it involves the lips, tongue, mouth, or throat and interferes with breathing.
• Belly pain or cramps.
• Nausea, vomiting, or diarrhea.
• Low blood pressure, shock, and unconsciousness.

The sooner symptoms occur after exposure to the substance, the more severe the anaphylactic reaction is likely to be. An anaphylactic reaction may occur with the first exposure to an allergen, with every exposure, or after several exposures. An anaphylactic reaction can be life-threatening and is a medical emergency. Emergency care is always needed for an anaphylactic reaction.