Allergic disease and anaphylaxis

Allergy is an immune-mediated hypersensitivity to an otherwise tolerated antigen. IgE-mediated reactions arise when allergen cross-links mast-cell or basophil-bound IgE, releasing histamine, tryptase, lipid mediators and cytokines. Vascular permeability, vasodilation, bronchoconstriction and mucosal oedema explain urticaria, hypotension, wheeze and upper-airway symptoms. Non-IgE mechanisms and direct mast-cell activation can produce clinically similar acute reactions, so emergency recognition depends on physiology rather than proving mechanism.

Anaphylaxis is likely when illness begins suddenly and threatens airway, breathing or circulation, usually with skin or mucosal features. Airway problems include swelling, hoarse voice or stridor; breathing problems include wheeze, hypoxaemia or exhaustion; circulation problems include hypotension, collapse or shock. Gastrointestinal symptoms can occur, especially after food exposure, but isolated rash or isolated nausea without ABC compromise is not anaphylaxis. Skin signs may be absent, and a normal blood pressure does not exclude respiratory anaphylaxis.

Immediate management follows ABCDE. Call for emergency help and stop exposure when this can be done safely. Position matters: sudden standing or walking can worsen venous return; lie the patient flat with legs raised, adapting position for respiratory distress, pregnancy or unconsciousness. Give intramuscular adrenaline 1 mg/mL in the anterolateral thigh. For adults and children older than 12 years, the RCUK healthcare dose is 500 micrograms (0.5 mL); repeat after 5 minutes if ABC problems persist.

Age determines paediatric dosing: 6–12 years 300 micrograms (0.3 mL); 6 months–6 years 150 micrograms (0.15 mL); under 6 months 100–150 micrograms (0.1–0.15 mL), all using 1 mg/mL adrenaline intramuscularly. These are healthcare-provider ampoule-and-syringe doses, not automatic substitution for an individual’s prescribed auto-injector device. Intravenous adrenaline is hazardous and reserved for experienced specialists in a monitored setting. For hypotension or shock, RCUK uses a rapid isotonic crystalloid bolus of 500–1,000 mL in an adult or 10 mL/kg in a child, with response and overload reassessment before further fluid.

After stabilisation, reconsider mimics such as vasovagal syncope, asthma, panic, sepsis and cardiogenic events, but do not delay adrenaline when anaphylaxis with ABC compromise is likely. In adults and young people aged 16 or over, take one mast-cell tryptase sample as soon as possible after emergency treatment starts and a second ideally 1–2 hours, but no later than 4 hours, from symptom onset. In children under 16, consider timed sampling when venom, medicine or idiopathic anaphylaxis is suspected. Arrange a convalescent baseline at specialist follow-up. Sampling never delays resuscitation, and a normal result does not exclude anaphylaxis.

NICE NG258 uses recovery-based observation. A suitably qualified, experienced clinician may consider discharge after 2 hours from resolution of airway swelling and return of normal breathing, blood pressure and heart rate only if all low-risk conditions hold: one IM adrenaline dose was given within 30 minutes of onset with a good response in 5–10 minutes; symptoms have fully resolved; the person already has two in-date auto-injectors and knows their use; and appropriate adult supervision is available if needed. Observe at least 6 hours after all symptoms resolve following two doses or a previous biphasic reaction. Observe at least 12 hours after all symptoms resolve for more than two doses, severe asthma or severe respiratory compromise, continuing allergen absorption, out-of-hours presentation, inability to respond to deterioration, or difficult emergency access. Under-16s who do not meet the early-discharge criteria need inpatient paediatric care. A separate supervised-allergy-challenge exception permits experienced clinicians to consider 2-hour observation after resolution even following two IM doses.

Long-term care addresses recurrence. Record the suspected trigger, route, latency, symptoms, physiology and response rather than entering an unverified broad allergy label. After emergency treatment for suspected anaphylaxis, offer specialist allergy referral. At discharge ensure two in-date adrenaline auto-injectors, device-specific training with return demonstration and advice to carry both, except when the cause is an easily avoided drug allergy. Provide written emergency and avoidance advice and review cofactors such as exercise, alcohol, infection and medicines that can amplify reactions.