What is type IV HS?

➢ Unlike the other types, it is not antibody mediated but rather is a type of cell-mediated response. ➢ Type IV HS (DTH) represents immune reactivity where T lymphocytes have had prior encounter with antigen; it is mediated by antigen-specific effector T cells and monocytes/macrophages rather than by antibodies ➢ This reaction is caused when CD4+ Th1 helper T cells recognize foreign antigen in a complex with the MHC class II on the surface of antigen-presenting cells. ➢ These can be macrophages that secrete IL-12, which stimulates the proliferation of further CD4+ Th1 cells ➢ CD4+ T cells secrete IL-2 and interferon gamma (IFN-γ), inducing the further release of other Th1 cytokines, thus mediating the immune response ➢ Another possibility is that if APC presents Ag along with MHCI, then CD8+ cells are stimul