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Class 4 (Severe) Allergic Reactions: Anaphylaxis Recognition & Emergency Response

Class 4 hypersensitivity reactions are immediate, IgE-mediated, systemic, and potentially fatal. The clinical manifestation is anaphylaxis—a rapid, multi-organ allergic emergency requiring immediate epinephrine.

Anaphylaxis Diagnostic Criteria (Any 1 of 3)

  1. Acute onset (minutes–hours) with skin/mucosal signs (hives, angioedema, flush) PLUS respiratory compromise (wheeze, stridor, hypoxia) OR reduced BP (hypotension, syncope).
  2. Two or more of: skin/mucosal signs, respiratory compromise, reduced BP, persistent GI symptoms (crampy pain, vomiting) after likely allergen exposure.
  3. Reduced BP after known allergen exposure (systolic <90 mmHg or >30% drop from baseline).

Key Clinical Manifestations

SystemSigns/Symptoms
Cutaneous (80–90%)Urticaria (hives), angioedema (lips, tongue, eyelids), flushing, pruritus
RespiratoryLaryngeal edema (stridor, hoarseness, "throat closing"), bronchospasm (wheeze, dyspnea), hypoxia
CardiovascularHypotension, tachycardia, dysrhythmias, cardiovascular collapse, syncope
GastrointestinalNausea, vomiting, abdominal cramps, diarrhea
NeurologicSense of doom, confusion, lethargy (from hypoxia/hypotension)

Immediate Emergency Response

  1. EPINEPHRINE IM (anterolateral thigh) – FIRST AND MOST CRITICAL.
    • Adults: 0.3–0.5 mg (1:1000) IM
    • Children: 0.01 mg/kg (max 0.3 mg) IM
    • Repeat every 5–15 min if no improvement.
  2. Call 911 / Activate EMS. Even if symptoms improve—biphasic reactions occur in 20%.
  3. Position: Supine with legs elevated (if tolerant); left lateral if pregnant/vomiting. Do not stand/walk.
  4. Adjuncts (after epinephrine):
    • Oxygen (high flow)
    • IV Fluids (1–2 L NS bolus for hypotension)
    • Inhaled Beta-agonist (Albuterol) for bronchospasm
    • H1 Antihistamine (Diphenhydramine 25–50 mg IV/IM) – not a substitute for epinephrine
    • H2 Blocker (Famotidine 20 mg IV)
    • Corticosteroid (Methylprednisolone 125 mg IV) – prevents late-phase/biphasic reaction
  5. Observation: Minimum 4–6 hours (longer if severe/biphasic risk).

Common Triggers

  • Foods: Peanuts, tree nuts, shellfish, fish, milk, egg, wheat, soy, sesame
  • Medications: Penicillins, NSAIDs, contrast media, biologics
  • Insect venom: Bee, wasp, hornet, fire ant
  • Latex, Exercise-induced (food-dependent)

Long-Term Management

  • Epinephrine auto-injector prescription (2 devices) + training.
  • Allergist referral for trigger confirmation (skin test, IgE, component testing).
  • Allergen avoidance plan + written Anaphylaxis Action Plan.
  • Medical alert identification.
  • Venom immunotherapy (VIT) for insect sting anaphylaxis (98% effective).
  • Oral immunotherapy (OIT) for select food allergies (specialized centers).

Epinephrine first, epinephrine fast. Delay increases fatality risk.

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