Anaphylaxis is a severe and rapid systemic allergic reaction to a trigger substance, called an allergen. Minute amounts of trigger substances may cause a life-threatening anaphylactic reaction. Anaphylaxis may occur after ingestion, inhalation, skin contact or injection of a trigger substance. The most severe type of anaphylaxis - anaphylactic shock - will usually result in death if untreated.
Anaphylactic shock is a life-threatening medical emergency because of rapid constriction of the airway, often within minutes of onset. Calling for help immediately can save precious minutes. First aid for anaphylactic shock consists of obtaining advanced medical care at once; rescue breathing (a skill which is part of CPR) is likely to be ineffective but should be attempted if the victim stops breathing. The patient may have been diagnosed with anaphylaxis in the past, and could be carrying an Epi-pen (or similar devices) that could be available for immediate administration of epinephrine (adrenaline) by a layperson. Repetitive administration is only dangerous when done in rapid succession. Pulse rates in double-administration cases have been known to cause tachycardia (rapid heartbeat) and occasionally ventricular tachycardia with rates up to 240 beats/min. Nevertheless, if this prevents worsening of the airway constriction, it may still be life-saving.
Symptoms can include respiratory distress, hypotension (low blood pressure), fainting, unconsciousness, urticaria (hives), angioedema (swelling of the face, neck and throat) and itching. The symptoms 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). Histamine induces, beside other effects, vasodilation and bronchospasm (constriction of the airways).
Common causative agents in humans include food ingredients (nuts, peanuts, fruits), drugs (e.g. penicillin, contrast media, NSAIDs such as ibuprofen or diclofenac), latex, bee or wasp stings, and exercise (see exercise-induced anaphylaxis). Transfusion of incompatible blood products may cause an equivalent picture.
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, tracheotomy may be required to maintain oxygenation.
The clinical teatment of anaphylaxis by a doctor and in the hospital setting aims to treat the cellular hypersensitivity reaction as well as at the symptoms. Antihistamine drugs (which inhibit the effects of histamine at histamine receptors) are given but 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, brochodilator drugs are used. In severe cases, immediate treatment with epinephrine may be lifesaving. Supportive care with mechanical ventilation may be required.