Type 4 Hypersensitivity Reactions

Summary

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
      • Direct cell cytotoxicity by Cytotoxic T-cells
      • Inflammatory cytokine release by Helper T-cells
  • 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)