Anaphylaxis — Causes, Symptoms, Emergency Treatment & Prevention Guide — Symptoms, Causes & Treatment | MyMedicPlus

Anaphylaxis is a severe, potentially fatal systemic allergic reaction involving multiple organ systems simultaneously. It occurs when the immune system is re-exposed to a trigger allergen, causing massive mast cell and basophil degranulation with release of histamine, tryptase, leukotrienes, and prostaglandins. This cascade produces rapid vasodilation, bronchospasm, and mucosal oedema. Without immediate treatment with intramuscular adrenaline, anaphylaxis can be fatal within minutes. Globally, anaphylaxis affects approximately 0.5–2% of people over a lifetime, with incidence rising over recent decades. Approximately 1–3 people per million die from anaphylaxis annually, though the true mortality rate is likely under-reported due to misclassification. The UK Anaphylaxis Register reports approximately 20 deaths per year from anaphylaxis in the UK alone, the majority from food allergy. Anaphylaxis is more common in atopic individuals and those with prior anaphylactic reactions, though it can occur in anyone without prior history of allergy, emphasising the importance of broad public awareness and the wide availability of adrenaline auto-injectors in public settings.

The most common triggers are foods (peanuts, tree nuts, shellfish, fish, milk, eggs, wheat — accounting for 33–56% of cases), insect venom (bee and wasp stings — particularly in adults), medications (penicillin and beta-lactam antibiotics, NSAIDs, radiocontrast agents, neuromuscular blocking agents, chemotherapy), and latex. Exercise-induced anaphylaxis can occur with or without food cofactors. Risk factors for severe or fatal anaphylaxis include prior anaphylaxis, asthma or chronic lung disease, cardiovascular disease, mastocytosis (elevated baseline tryptase), and beta-blocker or ACE inhibitor use — beta-blockers antagonize the effects of adrenaline, making treatment more difficult. Idiopathic anaphylaxis (no identifiable cause despite full investigation) accounts for 20–30% of adult cases.

Anaphylaxis typically develops within minutes of allergen exposure. Skin and mucosal symptoms (present in 80–90% of cases): urticaria (hives), angioedema, flushing, pruritus. Respiratory symptoms: bronchospasm, wheeze, stridor, hoarse voice, dyspnoea — laryngeal oedema causing stridor is an emergency sign. Cardiovascular symptoms: hypotension, tachycardia, dizziness, collapse, loss of consciousness — cardiovascular collapse (anaphylactic shock). Gastrointestinal symptoms: nausea, vomiting, abdominal cramps, diarrhoea. Neurological: confusion, anxiety, sense of impending doom. Anaphylaxis can occur without skin symptoms — isolated cardiovascular or respiratory collapse — requiring high clinical suspicion especially in medication or anaesthesia-related reactions. Symptoms can progress from mild skin reactions to life-threatening cardiovascular and respiratory collapse within minutes. Any two or more organ system involvement following allergen exposure should be treated as anaphylaxis without delay, and adrenaline administered immediately.

Anaphylaxis is a clinical diagnosis based on the characteristic rapid-onset multi-system reaction following allergen exposure. The Brighton Collaboration and WAO clinical criteria require involvement of skin/mucosa plus at least one of: respiratory compromise, hypotension, or end-organ dysfunction. Serum tryptase drawn at 1–3 hours after reaction onset — elevated levels (above 20 mcg/L or above 1.2 x baseline + 2) support the diagnosis. Tryptase returns to baseline within 6–8 hours, so timing of blood draw is critical. After recovery (4–6 weeks), allergy investigation includes skin prick testing and specific IgE (radioallergosorbent tests — RAST) to identify the causative allergen. Allergy testing must NEVER be performed during or immediately after anaphylaxis. Baseline tryptase should be measured in all patients to screen for underlying mastocytosis.

Intramuscular adrenaline (epinephrine) 0.3–0.5 mg (or 0.15 mg for children under 25 kg) into the outer mid-thigh is the definitive first-line treatment and must be given immediately — any delay worsens prognosis. Call emergency services immediately (999/112/911). Position: supine with legs elevated unless respiratory distress requires sitting upright; avoid standing the patient up suddenly. Supplemental high-flow oxygen, IV crystalloid fluids (1–2 L bolus for hypotension). Antihistamines (chlorphenamine H1 blocker + ranitidine H2 blocker) and corticosteroids (hydrocortisone 200 mg IV or prednisolone 40 mg oral) are useful adjuncts but are NOT first-line and must not delay adrenaline administration. Observe for 4–12 hours post-reaction to monitor for biphasic anaphylaxis (recurrence in 5–20% at 1–72 hours). Discharge with two prescribed adrenaline auto-injectors (EpiPen or Jext) plus a written emergency action plan.

Anaphylaxis that is untreated or inadequately managed with delayed epinephrine carries a high risk of fatal outcome. Laryngeal oedema causing complete airway obstruction is the most common cause of death in fatal anaphylaxis, followed by cardiovascular collapse. Even non-fatal anaphylaxis without epinephrine can cause prolonged hypoxia leading to hypoxic brain injury, myocardial infarction (demand ischaemia from severe hypotension), and acute kidney injury. Biphasic anaphylaxis occurs in 5-20% of patients — a second, sometimes more severe wave of symptoms 4-72 hours after the initial reaction without re-exposure — underscoring the importance of at least 4-6 hours of hospital observation after every anaphylactic episode. Patients who carry but do not use their epinephrine auto-injector promptly face significantly worse outcomes. Psychologically, anaphylaxis survivors frequently develop food anxiety, social avoidance, and significant reduction in quality of life — up to 50% experience anxiety disorders. Without formal allergy investigation and immunotherapy where available (for venom anaphylaxis), the risk of recurrent potentially fatal reactions persists.

Strict avoidance of identified allergen triggers is the cornerstone of anaphylaxis prevention. Always carry two adrenaline auto-injectors at all times and know how to use them. Wear a medical alert bracelet or necklace identifying known allergens. For food allergy: read all food labels meticulously, declare allergy to restaurants and catering staff, be vigilant about cross-contamination. For insect venom allergy: venom immunotherapy (VIT) for bee or wasp sting allergy is highly effective — reducing the risk of future anaphylaxis by over 95% — and is recommended for adults with venom allergy. Develop and rehearse a personalised emergency action plan with family, carers, schools, and workplaces. Seek annual review with an allergist to assess ongoing risk and review immunotherapy suitability.

Anaphylaxis is a medical emergency — call 999/112/911 immediately and administer adrenaline without delay if symptoms suggest anaphylaxis: sudden urticaria with throat tightening, wheeze, or collapse following allergen exposure. Use the EpiPen immediately and call for emergency help even if symptoms initially appear mild. Always attend emergency services after using an EpiPen — effects last only 15–20 minutes and a second dose may be needed. After any anaphylactic episode, seek urgent referral to an allergy clinic for full allergen investigation. Consult your GP or allergist before travelling, for reviewing your emergency plan, when changing medications, or if venom immunotherapy is being considered. Do not wait for a second reaction — act on the first episode.