Hypersensitivity Reactions and Anaphylaxis: Recognition, Resuscitation and Emergency Management

Learning objectives

• Classify hypersensitivity reactions and distinguish common allergic symptoms from anaphylaxis.
• Recognise airway, breathing and circulation threats, including anaphylaxis without skin signs.
• Give immediate first aid and intramuscular adrenaline safely, with appropriate positioning and repeat dosing.
• Manage bronchospasm, shock, angio-oedema, refractory reactions and cardiac arrest with senior support.
• Plan monitoring, observation, discharge education, trigger avoidance, referral and documentation.

What is hypersensitivity?

Hypersensitivity is an exaggerated or inappropriate immune response to an antigen. It may be allergic (often IgE-mediated) or non-IgE-mediated, such as direct mast-cell activation by some medicines, radiocontrast or physical triggers. The emergency practitioner treats the physiology rather than waiting to prove the mechanism.

Classification of hypersensitivity

Common triggers and risk factors

• Foods such as nuts, seeds, eggs, milk, fish, shellfish and hidden ingredients.
• Medicines, especially antibiotics, analgesics, anaesthetic agents, vaccines and biologic products.
• Bee, wasp and other insect venom; latex; blood products; antiseptics and occupational exposures.
• Exercise, cold, heat, pressure, alcohol or co-factors that amplify a food or drug reaction.
• Asthma, mast-cell disorders, cardiovascular disease, beta-blocker/ACE-inhibitor use and delayed access to care can increase severity.
• A previous mild reaction does not guarantee a future reaction will be mild; ask specifically about previous airway or shock symptoms.

Clinical patterns

Mild allergic reaction

Localised itching, flushing, sneezing, mild nasal symptoms, limited urticaria or mild gastrointestinal discomfort without airway, breathing or circulation compromise. Observe closely because progression is possible.

Urticaria

Transient, raised, itchy wheals that migrate and fade, usually within 24 hours at each site. Urticaria alone is not anaphylaxis, but it may accompany it.

Angio-oedema

Deeper swelling of lips, eyelids, tongue, face, hands, feet or genitalia. Hoarseness, drooling, stridor, tongue swelling or difficulty swallowing signals airway danger. ACE-inhibitor angio-oedema may occur without hives and responds less reliably to antihistamines.

Anaphylaxis

A rapidly developing, life-threatening reaction involving airway, breathing or circulation, often with skin/mucosal changes but sometimes without them. Gastrointestinal symptoms can be prominent after food exposure.

Anaphylaxis recognition criteria

• Two or more systems affected rapidly after exposure: skin/mucosa plus respiratory, cardiovascular or persistent gastrointestinal symptoms.
• Acute hypotension or collapse after a known allergen, even without skin signs.
• Severe bronchospasm after exposure in a patient with asthma, even without rash.

Immediate scene and first contact actions

1. Recognise the emergency, call for help and bring the resuscitation trolley, oxygen and defibrillator.
2. Remove the trigger if possible: stop an infusion, remove a stinger without squeezing the venom sac, stop food or medicine exposure.
3. Lay the patient flat with legs elevated; allow a patient with severe breathing difficulty to sit with legs extended, and place a pregnant patient on the left side. Never allow standing or walking.
4. Give intramuscular adrenaline into the anterolateral middle third of the thigh as soon as anaphylaxis is suspected. Do not delay for IV access or antihistamines.
5. Record time, dose and response. Repeat according to the local protocol—commonly after about five minutes if airway, breathing or circulation problems persist.
6. Transfer every anaphylaxis patient to an appropriate clinical setting for monitoring, even if symptoms improve.

ABCDE emergency assessment

A — Airway

• Look for tongue/lip swelling, hoarseness, stridor, drooling, inability to swallow or a muffled voice.
• Call anaesthesia/airway help early. Prepare suction, oxygen, bag-mask ventilation and a difficult-airway plan.
• Airway swelling can progress after initial improvement; reassess repeatedly.

B — Breathing

• Assess respiratory rate, work, wheeze, silent chest, SpO2, cyanosis and ability to speak.
• Give high-concentration oxygen for hypoxaemia or severe reaction and use bronchodilator therapy for persistent bronchospasm after adrenaline.
• Be alert for upper-airway obstruction, pulmonary oedema, aspiration and fatigue.

C — Circulation

• Check pulse, blood pressure, capillary refill, mental state, skin temperature and urine output.
• Establish IV or IO access, obtain ECG and give warmed isotonic crystalloid boluses for shock with repeated reassessment.
• If shock persists after IM adrenaline and fluids, call critical care for an adrenaline infusion; IV adrenaline is for experienced specialists with continuous monitoring.

D — Disability

• Assess GCS/AVPU, glucose, agitation, collapse and seizure activity.
• Confusion or drowsiness can indicate cerebral hypoperfusion or hypoxia.

E — Exposure

• Inspect skin, mucosa, injection sites, stings and medical devices; look for urticaria, flushing, pallor or angio-oedema.
• Prevent heat loss and examine for alternative causes such as sepsis, asthma, pulmonary embolism or vasovagal syncope.

Adrenaline: first-line treatment

Intramuscular adrenaline (epinephrine) is the life-saving first-line drug for anaphylaxis. Use the concentration and age/weight-based dose in the current local protocol. A commonly used professional reference is 1 mg/mL (1:1000) IM into the anterolateral thigh—adult dose often 0.5 mg and child dose approximately 0.01 mg/kg up to the local maximum. Repeat after about five minutes when airway, breathing or circulation problems persist. There is no absolute contraindication to IM adrenaline in life-threatening anaphylaxis.

Adjunctive treatment after adrenaline

• Oxygen: give high concentration for hypoxia, shock or severe respiratory symptoms; titrate once stable.
• Fluids: give rapid isotonic crystalloid boluses in hypotension or poor perfusion, reassessing lungs, blood pressure and capillary refill.
• Bronchodilator: inhaled salbutamol for persistent bronchospasm after adrenaline; do not use it instead of adrenaline.
• Antihistamine: may relieve itch and hives after stabilisation, but does not treat airway swelling, bronchospasm or shock and must never delay adrenaline.
• Corticosteroid: not a first-line resuscitation medicine; consider only under senior/local guidance for refractory asthma or selected reactions.
• Glucagon: specialist advice may be needed for refractory anaphylaxis in a patient taking a beta-blocker.

Refractory anaphylaxis and cardiac arrest

1. Confirm the diagnosis, correct positioning and repeat IM adrenaline at the protocol interval.
2. Call critical care, obtain IV/IO access, give cautious crystalloid and identify ongoing exposure.
3. Start a titrated adrenaline infusion only with an experienced clinician, infusion pump and continuous ECG, blood-pressure and oxygenation monitoring.
4. Use advanced airway and ventilation support if needed; prepare for a difficult airway with swelling.
5. If cardiac arrest occurs, start high-quality CPR and follow the local ALS algorithm; give IV/IO adrenaline according to cardiac-arrest protocol, not the IM anaphylaxis dose.

Special situations

• Pregnancy: treat maternal anaphylaxis promptly; place left lateral or manually displace the uterus and involve obstetric and neonatal teams.
• Children: use weight-based dosing, age-appropriate equipment, maintenance fluids and early paediatric support.
• Asthma: wheeze after allergen exposure should be treated as anaphylaxis until proven otherwise; adrenaline comes before inhalers.
• ACE-inhibitor angio-oedema: stop the drug, assess the airway early and involve anaesthesia; absence of hives does not make it safe.
• Venom sting: remove the stinger if visible, treat anaphylaxis, observe for recurrence and refer for venom immunotherapy assessment when indicated.
• Infusion or transfusion reaction: stop the product, maintain IV access with compatible fluid, notify the blood bank/team and follow the transfusion-reaction pathway.

Differential diagnosis

Investigations and observation

• Diagnosis is clinical; do not delay treatment for tests.
• Obtain ECG, glucose, electrolytes, blood gas/lactate and other tests according to severity and differential.
• Serum tryptase may support later specialist investigation when collected at the protocol-defined times; a normal result does not exclude anaphylaxis.
• Observe in a monitored setting. Longer observation or admission is appropriate after repeated adrenaline, severe respiratory/circulatory compromise, delayed presentation, asthma, mast-cell disease, uncertain trigger or poor access to emergency care.
• Watch for biphasic reactions—recurrent symptoms after initial resolution without a new exposure.

Nursing interventions

• Record time of onset, suspected trigger, ABCDE findings, adrenaline dose/time/site and response.
• Keep the patient supine or in the safest breathing position, never allow unaided standing, and reassure continuously.
• Monitor ECG, SpO2, respiratory rate, blood pressure, mental state, capillary refill and urine output.
• Prepare repeated adrenaline, oxygen, suction, nebuliser, IV fluids, airway equipment and defibrillator.
• Check all medicines for duplicate or hidden allergens and document adverse reactions prominently.
• Provide privacy, explain procedures and support the family without delaying resuscitation.
• Before discharge, teach auto-injector technique, trigger avoidance, emergency action plan and the need to call for help after use.

Observation, discharge and follow-up

Prevention and patient education

• Provide a written emergency plan describing symptoms, adrenaline use and when to repeat it or call for help.
• Teach avoidance of the identified food, medicine, venom or latex and how to read labels or communicate the allergy.
• Carry two adrenaline auto-injectors when prescribed, check expiry dates and store them according to manufacturer guidance.
• Tell every healthcare worker about the reaction; wear a medical-alert card or bracelet.
• Refer for supervised allergy testing, venom immunotherapy or drug-allergy assessment when appropriate.
• Review asthma control and other co-factors that increase risk.

Scenario-based application

Common errors to avoid

• Waiting for a rash, laboratory result or specialist review before giving IM adrenaline.
• Allowing a recovering patient to stand or walk, causing sudden cardiovascular collapse.
• Using antihistamines or steroids as substitutes for adrenaline.
• Giving a dangerous IV adrenaline bolus from a 1:1000 ampoule outside expert critical care.
• Failing to repeat adrenaline when airway, breathing or circulation problems persist.
• Missing anaphylaxis in asthma, pregnancy, children, dark skin or patients without urticaria.
• Discharging without an action plan, trigger documentation, auto-injector teaching and referral.

Documentation checklist

• Time, trigger, route of exposure and previous allergic reactions.
• ABCDE findings, vital-sign trends, skin/mucosal features and differential diagnosis.
• Adrenaline concentration, dose, route, site, time and response; fluids, oxygen and adjuncts.
• Airway/critical-care calls, observation period, investigations and disposition.
• Allergy alert, pharmacovigilance report, action plan, device teaching and follow-up.

Quick revision questions

1. What is the difference between urticaria, angio-oedema and anaphylaxis?
2. Can anaphylaxis occur without skin signs? Explain.
3. What is the first-line medicine, route and preferred injection site?
4. How should a patient with anaphylaxis be positioned, and why must they not stand?
5. When should IM adrenaline be repeated?
6. Which treatments are adjuncts and must never replace adrenaline?
7. When is an adrenaline infusion appropriate?
8. What education and equipment are required before discharge?

Key takeaways

Further reading

• Resuscitation Council UK: Anaphylaxis guidance
• NHS: Anaphylaxis
• WHO emergency treatment guidance for children
• World Allergy Organization anaphylaxis guidance