Epidemiology
Etiology
Pathophysiology
Compared to type I-III hypersensitivity reactions, which are antibody-mediated, type IV reactions are mediated by T cells. Type IV hypersensitivity reactions involve two major steps:
- T cell sensitization: skin penetration by the antigen → uptake of the antigen by Langerhans cell → migration to lymph nodes → formation of sensitized T lymphocytes
- Presensitized T cell response (after repeated contact with the antigen)
- CD4+ T cells recognize antigens on antigen-presenting cells → release of inflammatory lymphokines cytokines (e.g., IFN-γ, TNF-α) → macrophages activation → phagocytosis of target cells
- CD8+ T cells recognize antigens on somatic cells → cell-mediated cytotoxicity → direct cell destruction
- CD4+ T cells recognize antigens on antigen-presenting cells → release of inflammatory lymphokines cytokines (e.g., IFN-γ, TNF-α) → macrophages activation → phagocytosis of target cells
Examples
- Skin tests
- Candida skin test (to test the immune function of T cells)
- A diagnostic test in which Candida albicans antigen is injected intradermally to the arm. The injection site is examined 48 hours later. Induration ≥5 mm in diameter is considered a positive reaction and indicates prior exposure to antigen and an intact immune response mediated by T cells.
- This test requires patients to have encountered Candida before. Luckily, nearly everyone encounters Candida at some point in their lives.
- Mantoux tuberculin skin test for latent tuberculosis
- Candida skin test (to test the immune function of T cells)