How is mechanism of Type IV HS?

Activated CD8+ T cells destroy target cells on contact, whereas activated macrophages produce hydrolytic enzymes and, on presentation with certain intracellular pathogens, transform into multinucleated giant cells ➢ The DTH reaction involves cellular activation of T helper cells (CD4+) and/or cytotoxic T cells (CD8+CTLs). ➢ Subsequent cytokine secretion gives rise to distinct pathologies ➢ It is termed ‘delayed‐type’ HS because the reaction usually manifests 12–24 h (two to three days) post antigen exposure. ➢ Type IV reactions not only develop slowly but, depending on whether the antigen persists or is removed, they can be prolonged or relatively transient