Sushi fans have many options for raw fish, but eel is always served cooked. Why is that? Does it taste bad?

Taste question aside, it can easily be lethal.

Eels’ blood is poisonous, which discourages other creatures from eating them. A very small amount of eel blood is enough to kill a person, so raw eel should never be eaten.

Their blood contains a toxic protein that cramps muscles, including the most important one, the heart.

Proteins are made of long chains of amino acids that fold together in a specific way that defines their shape, and from that, to a very large extent, their function. Cooking unfolds the proteins and renders them harmless, so you are fine eating cooked eel (which I am fond of).

Despite its lethal nature, eel toxin played a very important - and positive - role in the history of medicine: Charles Robert Richet, a Parisian doctor, used it to make an important discovery.

Louis Pasteur had shown that animals could acquire resistance to diseases such as cholera by being inoculated with weakened strains of the disease-causing bacteria.

Richet theorized that he might get a similar result with toxic substances by attempting to vaccinate an animal against a poison by giving it a small dose of the poison.

He tried out the idea with eel blood serum - basically blood without any cells or clotting factors - injected into dogs.

Rather than a protective effect, or prophylaxis, from the initial exposure, however, he found a greatly increased and often fatal reaction, an anaphylaxis.

This sort of reaction is also seen in people who have dangerous allergic reactions to things like bee stings. An understanding of the phenomenon is a vital part of immunology.

Richet earned the Nobel Prize for medicine in 1913 for this work.

Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to drknowledge@globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.

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