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What Is Anaphylaxis?

Anaphylaxis is a severe, potentially life-threatening allergic reaction. It can occur within seconds or minutes of exposure to something you’re allergic to, but can also occur hours later.

Anaphylaxis causes your immune system to release a flood of chemicals (histamine, basophils, mast cells, and other substances) that can cause you to go into shock — your blood pressure drops suddenly and your airways narrow, blocking breathing. Signs and symptoms include a rapid, weak pulse; a skin rash; and nausea and vomiting. Common triggers include certain foods, vaccines, some medications, insect venom, latex, even red meat.

Many people don’t know the story about how anaphylaxis was first discovered.

Literally meaning “without protection”, anaphylaxis was coined by Charles Richet in 1902, who later won the Nobel Prize for his discovery.

Richet and professor Paul Jones Portier accidentally discovered anaphylaxis on a marine expedition in the Atlantic after vaccinating dogs with toxins. The two French scientists immunized dogs via injections with a local anemone and jellyfish toxin, and were surprised to discover that upon subsequent injections with even smaller doses of toxin, the dogs experienced a violent reaction and died, whereas they tolerated their initial injection without distress.

Thus instead of inducing tolerance (prophylaxis), when lethal responses resulted from previously tolerated doses, he coined the word anaphylaxis – without protection. He was subsequently awarded the Nobel Prize in Physiology or Medicine.

Richet proposed the name “aphylaxis” (derived from the Greek: a, contrary to + phylaxis, protection), which was changed to “anaphylaxis” because “aphylaxis” lacked euphonic expression.

Their appreciation of uncovering a new phenomenon was described as the outcome of “administration of a substance insufficient to kill or even sicken a normal animal that produces fulminating symptoms and death in an animal previously inoculated with the same substance.”

They would later understand this initial injection was sensitizing the animal and priming their immune system for allergic response upon subsequent injection.

Although the notion of an allergy would be described two years after Richet and Portier’s discovery, they determined that anaphylaxis was an immune response, and their results were the first concrete evidence that immune reactions could have adverse effects.

By 1903 he had shown that any protein could produce the same effect if there were 3 to 4 weeks between injections.

In 1905 Clemens von Pirquet, an Austrian pediatrician, noticed that patients vaccinated for smallpox using horse serum reacted quickly and severely to a second dose. Pirquet correctly deduced that the symptoms of what he called serum sickness were being caused by the immune system producing antibodies to fight antigens, or foreign substances contained in the serum. In 1906, he coined a new term for this antibody-antigen interaction: allergy.

If you experience anaphylaxis or any reaction after a vaccine, please report your reaction to the Vaccine Adverse Events Reporting System, or


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