About This Page
This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
The Bottom Line
- Anaphylaxis = severe systemic allergic reaction with airway/breathing/circulation compromise — life-threatening
- Treatment: IM adrenaline 1:1000 (0.5 mg adult, 0.3 mg 6–12 years, 0.15 mg <6 years) into anterolateral thigh. Repeat every 5 min if no improvement
- ABCDE approach: remove trigger, lie flat (legs elevated unless respiratory distress), high-flow oxygen, IV fluid bolus if hypotensive
- After adrenaline: chlorphenamine (antihistamine) + hydrocortisone (prevent biphasic reaction)
- Serum tryptase: take at 1–2 h and at ≥24 h (baseline) post-event — confirms mast cell degranulation
- ALL patients must be referred to allergy clinic and prescribed two adrenaline auto-injectors (EpiPen/Jext)
Overview
Anaphylaxis is a severe, potentially life-threatening systemic hypersensitivity reaction characterised by rapid onset of airway, breathing, and/or circulatory problems, usually associated with skin and mucosal changes. It is typically IgE-mediated (type I hypersensitivity) causing widespread mast cell and basophil degranulation with release of histamine, tryptase, leukotrienes, and prostaglandins. The most common triggers are foods (peanuts, tree nuts, shellfish, milk, egg), drugs (antibiotics, NSAIDs, anaesthetic agents), and insect venom (wasp, bee stings). Latex and exercise are less common causes.
Epidemiology
Anaphylaxis incidence is approximately 50–100 per 100,000 person-years in the UK, and is increasing. Fatal anaphylaxis is rare (~20 deaths/year in the UK) but underreported. Food-related anaphylaxis is the most common trigger in children; drug-related is most common in adults. Risk factors for severe/fatal anaphylaxis include: coexisting asthma (especially poorly controlled), adolescents/young adults (risk-taking behaviour, delayed adrenaline use), concurrent beta-blocker or ACE inhibitor use, and previous severe reaction.
Clinical Features
Symptoms
Rapid onset (usually within minutes of exposure, up to 2 hours)
Skin: urticaria (hives), angioedema, generalised erythema, pruritus — present in >80%
Airway: throat tightness, voice change (hoarseness), stridor, tongue/lip swelling
Breathing: wheeze, dyspnoea, tachypnoea, respiratory arrest
Circulation: dizziness, presyncope, hypotension, tachycardia, cardiac arrest
GI: nausea, vomiting, abdominal pain, diarrhoea
Sense of impending doom
Signs
Urticaria and/or angioedema (may be absent in 10–20%)
Stridor (laryngeal oedema)
Bronchospasm: expiratory wheeze, silent chest in severe cases
Hypotension (systolic <90 mmHg), tachycardia
Altered consciousness, collapse
Cardiac arrest (PEA) in extreme cases
Investigations
First-line
Clinical diagnosisAnaphylaxis is diagnosed CLINICALLY — do NOT delay treatment for investigations
Serum mast cell tryptaseTake at: 1) as soon as feasible after resuscitation, 2) 1–2 hours post-onset (peak), 3) ≥24 hours (baseline). Elevated tryptase confirms mast cell activation
Second-line
Skin prick testingAllergy clinic — 4–6 weeks post-event. Identifies specific trigger allergen
Specific IgE blood testsAlternative to skin prick testing — measures allergen-specific IgE antibodies
Specialist
Component-resolved diagnosticsIdentifies specific allergenic proteins — helps distinguish true allergy from cross-reactivity
Drug provocation testingSpecialist supervised challenge — for suspected drug allergy when skin tests/IgE inconclusive
1
Immediate treatment (ABCDE)
- Call for help — this is a medical emergency
- Remove trigger if possible (stop infusion, remove stinger)
- IM ADRENALINE 1:1000 into anterolateral thigh — THE MOST IMPORTANT INTERVENTION
- Adult: 0.5 mg (500 mcg). Child 6–12 years: 0.3 mg. Child <6 years: 0.15 mg
- Repeat every 5 minutes if no improvement. There is NO contraindication to IM adrenaline in anaphylaxis
- Position: lie flat with legs elevated (if hypotensive). Sit up if respiratory distress predominates
- High-flow oxygen (15 L/min via non-rebreather)
- IV fluid bolus: 500 mL–1 L crystalloid stat if hypotensive (20 mL/kg in children)
2
Adjunctive medications (AFTER adrenaline)
- Chlorphenamine IV: adult 10 mg, child 6–12 years 5 mg, child 1–5 years 2.5 mg
- Hydrocortisone IV: adult 200 mg, child 6–12 years 100 mg, child 1–5 years 50 mg
- These are SECONDARY to adrenaline — do NOT give these instead of adrenaline
- Inhaled salbutamol if persistent bronchospasm despite adrenaline
3
Post-event
- Observe for biphasic reaction: minimum 6 hours (12 hours if severe, asthmatic, or late-onset reaction)
- Prescribe two adrenaline auto-injectors (EpiPen 300 mcg for adults, 150 mcg for children)
- Provide written anaphylaxis action plan
- Refer ALL patients to allergy clinic (NHS allergy service)
- Educate: trigger avoidance, auto-injector technique, medical alert bracelet
Complications
- Death: From airway obstruction or cardiovascular collapse — risk reduced by early adrenaline
- Biphasic reaction: Recurrence of symptoms 4–12 hours after initial resolution — occurs in ~5%. Hence minimum 6 h observation
- Refractory anaphylaxis: Not responding to repeated IM adrenaline — consider IV adrenaline infusion (specialist/anaesthetist only)
- Cardiac arrest: PEA arrest — follow ALS algorithm with adrenaline as per anaphylaxis protocol
- Psychosocial: Anxiety, food avoidance, reduced quality of life — particularly in children
UKMLA Exam Tips
- 1IM adrenaline 1:1000 (0.5 mg adult) is THE first-line treatment. IV adrenaline is only for cardiac arrest or refractory anaphylaxis (specialist only)
- 2Route is IM into anterolateral THIGH — not subcutaneous, not deltoid
- 3There is NO absolute contraindication to adrenaline in anaphylaxis — even in pregnancy, cardiac disease, or beta-blocker use
- 4Antihistamines and steroids are ADJUNCTS — they are NOT substitutes for adrenaline and should NEVER be given instead
- 5Tryptase: take 3 samples (ASAP, 1–2 h, and ≥24 h baseline). Elevated tryptase confirms the diagnosis retrospectively
- 6ALL patients get two adrenaline auto-injectors and allergy clinic referral — this is mandatory
- 7Biphasic reaction: ~5% recur at 4–12 h — this is why minimum 6 h observation is required
practicetest your knowledge on anaphylaxisApply what you've learnt with UKMLA-style questions from the iatroX Q-Bank — emergency medicine and beyond.
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