Medicine & USMLE

Type 4 Hypersensitivity Reactions

Hypersensitivity Reactions
  1. Type 1 Hypersensitivity Reactions
  2. Type 2 Hypersensitivity Reactions
  3. Type 3 Hypersensitivity Reactions
  4. Type 4 Hypersensitivity Reactions


Type 4 Hypersensitivity Reactions (T4HSR), also called cell-mediated hypersensitivity reactions, are a type of undesired immune response mediated by T-cells. Killer T-cells and Helper T-cells become activated to an antigen, resulting in an inflammatory response. Since T-cell activation and effects are slow (adaptive immune response), Type 4 hypersensitivity usually takes 48-72 hours to manifest symptomatically. This gives rise to its other name: delayed-type hypersensitivity. Some classic examples of Type 4 hypersensitivity include contact dermatitis and the PPD (Mantoux) or TB skin test.

Key Points

  • Type IV Hypersensitivity Reactions (T4HSR)
    • Undesired immune response mediated by T-cells
      • Also known as cell-mediated hypersensitivity
      • Damage mediated by two mechanisms
    • Reaction delayed 48-72 hours after exposure to antigen
      • Also known as delayed-type hypersensitivity
    • Examples
      • contact dermatitis (e.g. poison ivy, nickel allergy)
        • patch test for contact dermatitis
      • Graft vs. Host Disease
      • PPD Test for TB exposure
      • Candida skin test
        • Assesses T-cell activity (e.g. in AIDS)