Anaphylaxis is said to be present if a person is extremely sensitive to a substance on first exposure. If the person is exposed to the substance again, the result may be anaphylactic shock, an explosive overreaction of the body's immune system. Anaphylactic shock is a violent allergic reaction characterized by itching skin, hives, breathing trouble, collapse of the circulatory system, and sometimes vomiting, diarrhea, and cramps in the abdomen.

Causes

Anaphylaxis may be triggered by an insect sting; by a food allergy; by animal serum used in a vaccine; by desensitizing injections; or by one of various drugs. The allergen (the substance that sets off the reaction) is usually a protein, called an antigen. When it first enters the body, certain body cells treat it like an invading microorganism and create antibodies, which are protective substances that fight infection. On subsequent exposures to the same antigen, the antibodies are ready for the "invader" and stimulate the release of certain chemicals.

One of these chemicals, histamine, causes contraction of muscles in the digestive and respiratory tracts, resulting in abdominal cramps and wheezing. Histamine also causes the small blood vessels to enlarge and lose plasma (blood fluid) to surrounding tissues. The decrease in the volume of blood in the vessels results in a drop in blood pressure, which can lead to shock (collapse of the circulatory system).

Symptoms

Symptoms of anaphylactic shock develop rapidly. Within 15 minutes, the victim becomes uneasy, upset, and red in the face. Rapid heartbeat, prickling and itching sensations in the skin, throbbing in the ears, sneezing, coughing, and breathing difficulty are also likely. Vomiting, incontinence, and even convulsions may occur. Shock may result, in which blood vessels collapse, the pulse becomes weak and rapid, and the person becomes cold, clammy, and faint. Without immediate medical aid, anaphylactic shock may result in death.

Treatment

In cases of anaphylactic shock, medical aid should be obtained as quickly as possible -- call an emergency squad. Keep the patient's airway open. Have the patient lie down, with the legs elevated.

Immediate treatment with the drug epinephrine is critical. It counteracts the action of the histamine released in the blood, which is causing the symptoms of anaphylactic shock. In extreme cases, especially when shock has occurred or there is great breathing difficulty, intravenous solutions are given to restore blood volume and raise blood pressure. Anyone with a severe reaction is usually hospitalized for at least 24 hours.

Prevention

A preventive measure against anaphylactic shock is routine allergy skin testing. In addition, anyone who has had an allergic reaction to an insect sting should carry a kit containing a syringe of epinephrine and an epinephrine-filled nasal sprayer for prompt self-treatment. Desensitizing injections of the insect venom should also be considered to allow the body to build up a resistance to the venom.

Anaphylactoid Reactions

Similar to anaphylaxis in effect (but not in cause) are anaphylactoid reactions, which can occur on the first injection of certain drugs, such as morphine, and of contrast media (special solutions used when performing certain X-ray studies). These are not allergic reactions in the pure sense, but they are serious nevertheless.



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