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Anaphylaxis is a severe, potentially fatal systemic allergic reaction requiring immediate recognition and treatment. It is mediated by IgE-dependent mast cell and basophil degranulation and can progress from initial symptoms to cardiovascular collapse within minutes.
In IgE-mediated anaphylaxis, prior sensitization leads to allergen-specific IgE bound to mast cells. Re-exposure triggers crosslinking of IgE receptors and massive release of histamine, tryptase, leukotrienes, and prostaglandins. These mediators cause vasodilation, increased vascular permeability, bronchospasm, and mucus secretion. Non-IgE-mediated anaphylactoid reactions (contrast media, NSAIDs, opioids) activate mast cells directly, producing an identical clinical syndrome.
Clinical diagnosis requires rapid pattern recognition: acute onset affecting skin/mucosa (urticaria, angioedema) plus respiratory compromise or cardiovascular collapse, following exposure to a likely trigger. Intramuscular epinephrine to the anterolateral thigh is the first-line treatment — not antihistamines or steroids. The Sampson criteria define three clinical presentations. Biphasic reactions occur in 5-20% of cases, justifying a 4-6 hour observation period. Differentials include vasovagal syncope, panic attack, hereditary angioedema (no urticaria), and scombroid fish poisoning.
Common clinical reasoning questions about this topic
Anaphylaxis is diagnosed when acute onset illness involves skin/mucosal symptoms plus either respiratory compromise or cardiovascular collapse, OR when two or more body systems are affected after exposure to a likely allergen, OR when blood pressure drops after exposure to a known allergen.
Intramuscular epinephrine (0.3-0.5mg of 1:1000 solution) to the anterolateral thigh is the first-line treatment. Antihistamines and corticosteroids are adjuncts only and should never replace epinephrine.
Biphasic anaphylaxis occurs in 5-20% of cases when symptoms recur 1-72 hours after the initial reaction without re-exposure to the allergen. This is why a 4-6 hour observation period is recommended after anaphylaxis.