What is Anaphylaxis?
Anaphylaxis is a serious allergic reaction that is rapid in onset and may cause death. It arises as an acute, generalized IgE-mediated immune reaction involving specific antigen, mast cells and basophils. The reaction requires priming by the allergen, followed by re-exposure. To provoke anaphylaxis, the allergen must be systemically absorbed, either after ingestion or parenteral injection. A range of allergens that provoke anaphylaxis has been identified (find in the table).
Anaphylaxis is rare, and the symptom/sign constellation ranges from widespread urticaria to cardiovascular collapse, laryngeal oedema, airway obstruction and respiratory arrest leading to death:
Fatal reactions to penicillin occur once every 7.5 million injections.
Between 1 in 250 and 1 in 125 individuals have severe reactions to bee and wasp stings, and a death takes place every 6.5 million stings, with 60–80 deaths per year in North America, and 5–10 in the UK.
Central to the pathogenesis of anaphylaxis is the activation of mast cells and basophils, with systemic release of some mediators and generation of others. The initial symptoms may appear innocuous: tingling, warmth and itchiness. The ensuing effects on the vasculature give vasodilatation and oedema. The consequence of these may be no more than a generalized flush, with urticaria and angio-oedema. More serious sequelae are hypotension, bronchospasm, laryngeal oedema and cardiac arrhythmia or infarction. Death may occur within minutes. Serum platelet-activating factor (PAF) levels correlate directly with the severity of anaphylaxis whereas PAF acetylhydrolase (the enzyme that inactivates PAF) correlated inversely and was significantly lower in peanut sensitive patients with fatal anaphylactic reactions.
Early recognition and treatment are essential (See below table).
The best treatment is prevention. Avoidance of triggering foods, particularly nuts and shellfish, may require almost obsessive self-discipline. Patient education is necessary and many are instructed in the self-administration of adrenaline (epinephrine) and carry pre-loaded syringes. Desensitization has a well-established place in the management of this disorder, particularly if exposure is unavoidable or unpredictable, as in insect stings.