Hypersensitivity reactions are exaggerated immune responses to antigens that cause tissue injury and clinical illness. They are conventionally classified into four types (I-IV) by mechanism and timing.
In the emergency department the priority is rapid recognition and resuscitation of life‑threatening Type I (IgE‑mediated) reactions - anaphylaxis - while also recognising non‑immediate drug reactions, angioedema subtypes and the need for clear documentation and specialist follow‑up (NICE CG134; CG183; Resuscitation Council UK).
Classification - mechanisms and ED relevance
Key ED point: Type I reactions can progress rapidly and demand immediate ABC‑focused treatment. Types II-IV often present sub-acutely (commonly drug‑related) and require stopping the culprit agent, stabilisation and timely specialist referral (NICE CG134; CG183).
Type I hypersensitivity and anaphylaxis - recognition
Practical ED definition
- Suspect anaphylaxis when there is a rapid onset of airway and/or breathing and/or circulatory compromise after possible allergen exposure, with or without skin/mucosal signs. Do not exclude anaphylaxis if skin signs are absent (they may be absent in ≈20% of cases) (NICE CG134).
- Red flags: progressive facial/tongue/laryngeal swelling, stridor, severe bronchospasm not responding to bronchodilator, persistent hypotension or collapse.
Common precipitants
- Foods: nuts, shellfish, milk, eggs
- Drugs: β‑lactams, neuromuscular blockers, contrast media
- Insect venom, latex
- Exercise‑related or idiopathic triggers