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In medicine, anaphylaxis is a severe and rapid multi-system allergic reaction. Anaphylaxis occurs when a person is exposed to a trigger substance, called an allergen, to which they have already become sensitized. Minute amounts of allergens may cause a life-threatening anaphylactic reaction. Anaphylaxis may occur after ingestion, inhalation, skin contact or injection of an allergen. The most severe type of anaphylaxis—anaphylactic shock—will usually lead to death in minutes if left untreated.
The word is from New Latin (derived from Greek ἀνα-/ana, meaning "up, again, back, against") + φύλαξις/phylaxis, meaning "guarding, protection"—cf. prophylaxis).
Researchers typically distinguish between "true anaphylaxis" and "pseudo-anaphylaxis." The symptoms, treatment, and risk of death are identical, but "true" anaphylaxis is always caused directly by IgE-mediated degranulation of mast cells or basophils, and pseudoanaphylaxis is caused by any and all other means. The distinction is only important for researchers who are studying mechanisms of allergic reactions, and it often frustrates patients who feel they are being told that a life-threatening allergic reaction wasn't "real."
Anaphylactic shock, the most serious of allergic reactions, is a life-threatening medical emergency because of rapid constriction of the airway, often within minutes of onset. Calling for help immediately is important, as brain and organ damage rapidly occurs if the patient cannot breathe. Anaphylactic shock requires immediate advanced medical care; but other first aid measures include rescue breathing (part of CPR) and administration of epinephrine (adrenaline). Rescue breathing may be hindered by the constricted airways but is essential if the victim stops breathing on their own. If the patient has previously been diagnosed with anaphylaxis, they may be carrying an EpiPen (or similar device) for immediate administration of epinephrine (adrenaline) by a layperson to help keep the airway open. Use of an EpiPen or similar device will only provide temporary and limited relief of symptoms, so emergency medical services must still be contacted. Repetitive administration of epinephrine can cause tachycardia (rapid heartbeat) and occasionally ventricular tachycardia with heart rates potentially reaching 240 beats per minute, which can also be fatal. Extra doses of epinephrine can sometimes cause cardiac arrest. This is why some protocols advise intramuscular injection of only 0.3–0.5mL of a 1:1,000 dilution. The epinephrine will prevent worsening of the airway constriction, stimulate the heart to continue beating, and may be life-saving.
Epinephrine will act on Beta-2 adrenergic receptors in the lung as a powerful bronchodilator relieving allergic or histamine-induced acute asthmatic attack or anaphylaxis. Tachycardia results from stimulation of Beta-1 adrenergic receptors of the heart increasing contractility (inotropic effect) and frequency (chronotropic effect) and thus cardiac output.
Every patient prone to anaphylaxis should have an "allergy action plan" on file at school, home or in their office to aid family members, teacher and/or co-workers in case of an anaphylactic emergency. The Asthma and Allergy Foundation of America provides a free "plan" form anyone can print from their site. Action plans are considered essential to quality emergency care.
Symptoms of anaphylaxis are related to the action of immunoglobulin E (IgE) and other anaphylatoxins, which act to release histamine and other mediator substances from mast cells (degranulation). In addition to other effects, histamine induces vasodilation of arterioles and constriction of bronchioles in the lungs, also known as bronchospasm (constriction of the airways).
Symptoms can include the following:
- respiratory distress,
- hypotension (low blood pressure),
- urticaria (hives),
- flushed appearance,
- angioedema (swelling of the face, neck and throat),
- tears (due to angioedema and stress),
- itching, and
The time between ingestion of the allergen and anaphylaxis symptoms can vary for some patients depending on the amount of allergen ingested and sensitivity. Symptoms can appear immediately, or can be delayed by half an hour to several hours after ingestion. However, symptoms of anaphylaxis usually appear very quickly once they do begin.
Common causative agents in humans include:
- foods (e.g. milk, cheese, nuts, peanuts, soybeans and other legumes, fish and shellfish, wheat, fruit, and eggs);
- drugs (e.g. penicillins and cephalosporins, contrast media, ASA and other NSAIDs such as ibuprofen and diclofenac);
- Hymenoptera stings from insects such as bees, wasps, yellowjackets, hornets, and some stinging ants; and
- exercise (see exercise-induced anaphylaxis).
Transfusion of incompatible blood products may lead to extremely similar symptoms, albeit for substantially different biochemical reasons.
The anaphylactic reaction is mediated by antibodies. Anaphylactic sensitivity can be transferred to a normal (non-sensitive) recipient by means of serum containing such antibodies. This is then called passive transfer of the allergy or hypersensitivity.
The source of antibody may be from the same species, e.g. the human species, for which the predominant immunoglobulin involved is of the IgE class. Some known cases of deaths resulting from passive transfer involve organ transplants. It may also be transferred from animals of a different species. For example, a guinea pig may be sensitized by an intravenous injection of rabbit antibody to ovalbumin. When challenged (that is, brought in contact) with ovalbumin 48 hours later, the guinea pig will suffer fatal anaphylactic shock.
Paramedic treatment in the field may include injection with epinephrine, administration of oxygen therapy and, if necessary, intubation during transport to advanced medical care. In profuse angioedema, cricothyrotomy or tracheotomy may be required to maintain oxygenation.
The clinical treatment of anaphylaxis by a doctor and in the hospital setting aims to treat the cellular hypersensitivity reaction as well as the symptoms. Antihistamine drugs (which inhibit the effects of histamine at histamine receptors) are given but are usually not sufficient in anaphylaxis, and high doses of intravenous corticosteroids are often required. Hypotension is treated with intravenous fluids and sometimes vasoconstrictor drugs. For bronchospasm, bronchodilator drugs (e.g. Salbutamol, known as Albuterol in the United States) are used. In severe cases, immediate treatment with epinephrine can be lifesaving. Supportive care with mechanical ventilation may be required.
Patients must be monitored for four hours after being transported to medical care for the possibility of biphasic reactions (recurrence of anaphylaxis) .
Exercise-induced anaphylaxis shock occurs when a sufferer has eaten a food to which they are allergic to, followed by a high heart rate actively like jogging. This may be avoided by exercising on an empty stomach, drinking coffee for its caffeine, and taking vitamin C.
- Atopic syndrome
- Food allergy
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- Howland, R. 2006. Lippincott's Review of Pharmacology, 3rd Edition
- Gomella, LG. 2005. Clinician's Pocket Drug Reference. Drug Manual.