Anaphylaxis is a life-threatening type of allergic reaction.

Anaphylaxis is an severe, whole-body allergic reaction. After an initial exposure to a substance like bee sting toxin, the person's immune system becomes sensitized to that allergen. On a subsequent exposure, an allergic reaction occurs. This reaction is sudden, severe, and involves the whole body.

Tissues in different parts of the body release histamine and other substances. This causes constriction of the airways, resulting in wheezing, difficulty breathing, and gastrointestinal symptoms such as abdominal pain, cramps, vomiting, and diarrhea.

Histamine causes the blood vessels to dilate (which lowers blood pressure) and fluid to leak from the bloodstream into the tissues (which lowers the blood volume). These effects result in shock. Fluid can leak into the alveoli (air sacs) of the lungs, causing pulmonary edema.

Hives and angioedema (hives on the lips, eyelids, throat, and/or tongue) often occur. Angioedema may be severe enough to block the airway. Prolonged anaphylaxis can cause heart arrhythmias.

Some drugs (polymyxin, morphine, x-ray dye, and others) may cause an anaphylactoid reaction (anaphylactic-like reaction) on the first exposure. This is usually due to a toxic reaction, rather than the immune system mechanism that occurs with "true" anaphylaxis. The symptoms, risk for complications without treatment, and treatment are the same, however, for both types of reactions.

Anaphylaxis can occur in response to any allergen. Common causes include insect bites/stings, horse serum (used in some vaccines), food allergies, and drug allergies. Pollens and other inhaled allergens rarely cause anaphylaxis. Some people have an anaphylactic reaction with no identifiable cause.

Anaphylaxis occurs infrequently. However, it is life-threatening and can occur at any time. Risks include prior history of any type of allergic reaction.

Symptoms develop rapidly, often within seconds or minutes. They may include the following:

• Difficulty breathing
• Wheezing
• Abnormal (high-pitched) breathing sounds
• Confusion
• Slurred speech
• Rapid or weak pulse
• Blueness of the skin (cyanosis), including the lips or nail beds
• Fainting, light-headedness, dizziness
• Hives and generalized itching
• Anxiety
• Sensation of feeling the heart beat (palpitations)
• Nausea, vomiting
• Diarrhea
• Abdominal pain or cramping
• Skin redness
• Nasal congestion
• Cough

Examination of the skin may show hives and swelling of the eyes or face. The skin may be blue from lack of oxygen or may be pale from shock. Angioedema in the throat may be severe enough to block the airway.

Listening to the lungs with a stethoscope may reveal wheezing or indicate fluid (pulmonary edema). The pulse is rapid, and blood pressure may be low. Weakness, pale skin, heart arrhythmias, mental confusion, and other signs may indicate shock.

Testing for the specific allergen that caused anaphylaxis (if the cause is not obvious) is postponed until after treatment.