Hypersensitivity Reactions Types I-IV

A patient breaks out in hives 20 minutes after an antibiotic, but another develops a rash 48 hours after a TB skin test. Same immune system, completely different mechanisms — and your nursing response changes with the type.

Hypersensitivity reactions are classified into four types based on immune mechanism and timing. Type I (immediate/anaphylactic) is IgE-mediated, occurs within minutes, and includes anaphylaxis, allergic rhinitis, and asthma. Hallmarks: urticaria, bronchospasm, hypotension, laryngeal edema. Epinephrine IM is the first-line intervention — not diphenhydramine, not a steroid. Type II (cytotoxic) involves IgG or IgM antibodies attacking cell surfaces. Classic examples: hemolytic transfusion reactions, Rh incompatibility, heparin-induced thrombocytopenia. Onset is hours. Type III (immune complex) involves antigen-antibody complexes depositing in tissues, triggering inflammation. Think serum sickness (fever, joint pain, rash 1–3 weeks after exposure) and lupus. Type IV (delayed/cell-mediated) is T-cell driven with no antibody involvement. Onset is 24–72 hours. Examples: contact dermatitis, TB skin test reaction, and acute cellular transplant rejection. Nursing priorities shift by type: for Type I, maintain airway and administer epinephrine; for Type II, stop the offending agent immediately (blood product, medication); for Types III and IV, monitor for systemic inflammation and provide supportive care.

Key Distinctions

Don't confuse Type I (minutes, IgE, anaphylaxis) with Type IV (days, T-cells, no antibodies) — both involve allergens but timing and mechanism are opposite. Type IV is the only type not antibody-mediated — a classic board distractor. Students mix up Type II and Type III: Type II destroys specific cells directly (transfusion reaction), while Type III deposits complexes in tissues causing widespread inflammation (serum sickness). Epinephrine is first-line for anaphylaxis, not antihistamines — diphenhydramine is adjunct, not rescue.

Clinical Pearl

Think speed: Type I = minutes, Type IV = days. If the patient can't breathe, grab epinephrine first — antihistamines don't reverse airway compromise.