Hypersensitivity Types
Type I (IgE-mediated, immediate): anaphylaxis, allergic rhinitis, asthma. Type II (cytotoxic): transfusion reactions. Type III (immune complex): SLE, vasculitis. Type IV (delayed): contact dermatitis, TB test. Common allergens: peanuts, shellfish, latex, penicillin, NSAIDs, insect stings.
Anaphylaxis Recognition
Rapid onset (minutes to hours). Two+ systems: skin (urticaria, angioedema), respiratory (wheeze, stridor, dyspnea), cardiovascular (hypotension, tachycardia, collapse), GI (vomiting, cramping). Biphasic reactions: recurrence 1-72h after initial. Serum tryptase elevated 1-2h.
Emergency Management
Epinephrine 0.3-0.5 mg IM (anterolateral thigh) — first-line, do not delay. Repeat in 5-15 min. Position supine with legs elevated. Airway: oxygen, may need intubation. IV fluids 1-2 L bolus. Adjuncts: antihistamines, corticosteroids, bronchodilators. Observe 4-6h minimum.
Important Notes
- Epinephrine is first-line for anaphylaxis — antihistamines and steroids are adjuncts only, never substitutes.
- IM epinephrine in anterolateral thigh provides fastest absorption — not subcutaneous, not IV (unless cardiac arrest).
Clinical Tips
- Delay in epinephrine is leading cause of fatal anaphylaxis — give early.
- Always observe for biphasic reactions — minimum 4-6 hours.
Summary
Anaphylaxis requires immediate IM epinephrine, airway support, IV fluids, adjunct medications, observation for biphasic reactions, and epinephrine auto-injector prescription.
