A naphylactic shock is defined as a massive allergic reaction, but it seems like something else entirely. These incidents are sudden, bizarre and shocking. They involve deaths within minutes from substances such as peanuts or insect stings, which cause little or no harm to most people. Or, as in a case reported at an inquest in Birmingham last week, they involve hair dye.
Narinder Devi, a mother of three, was applying the dye, an enormously popular brand called Movida, when the reaction began. She had difficulty breathing and felt she was overheating, and ran into the bathroom to pour cold water over her head. Within an hour she had gone into cardiac arrest.
After Devi's death, the makers of Movida, Laboratoires Garnier, pointed out that such tragedies are very rare: it is estimated that there is one such death in the UK every 30 years or more. But another kind of severe allergic reaction to hair dye is less rare.
Alicia Richens, of St Andrews, Fife, was moved to tears when she read how Devi died. It is only a few weeks since Richens suffered a severe allergic reaction to a hair dye which left her face puffed up "like a blowfish".
She had followed the instructions by trying some of the dye on the back of her arm 48 hours before colouring her hair, with no adverse reactions. But in the days after she applied it to her hair she began to develop a rash and a feeling of being over heated.
"At first I though it was just a minor itchiness that would go away. By the third day, I couldn't stop scratching my scalp. Hives started appearing on various parts of my body and my face started swelling."
She recovered after a doctor prescribed a course of antihistamine pills, but has vowed never to dye her hair again. Devi suffered a "type one" allergic reaction, which starts within minutes and lasts less than an hour. Richens is thought to have had the other main type of reaction to hair dye: a "type four" reaction, which takes days to develop and lasts for days.
Such reactions to hair dyes happen in perhaps one in every 100,000 applications. In most cases, the component in the hair dye that prompts the reaction is para-phenylenediamine, known to be a potent cause of allergic rashes in susceptible people.
An allergy is what happens when the body's immune system becomes hyperactive, defending us from foreign bodies that are not a threat. When someone is exposed to a sensitising agent, this binds on to markers called E-class immunoglobins (IgEs), of which there tend to be more in allergy sufferers. The IgEs then latch on to a certain type of cell, which rupture and release histamines - irritating chemicals - into the body. This can cause local hives or, if there is widespread histamine release, anaphylaxis.
People die from anaphylaxis either because their blood pressure drops or because swelling in the throat blocks breathing. These processes can be reversed if adrenaline is injected.
According to medical experts, tests on small patches of skin - such as those advised by hair dye manufacturers - can give false results, as with Richens. Peter Lane, an immunologist at the University of Birmingham, says: "Usually, but not always, patients who develop anaphylaxis have a warning preliminary episode when they develop less severe symptoms.
"If you develop hives, or have any breathing difficulties or feel faint, stop using whatever provokes the reaction and seek medical advice. Your next exposure might be fatal."