Type IV Hypersensitivity

This reaction is called "delayed hypersensitivity" because it is mediated by sensitized CD4+ T lymphocytes which process antigens in association with class II HLA molecules and release lymphokines. The lymphokines promote a reaction (especially mediated through macrophages) beginning in hours but reaching a peak in 2 to 3 days.

This diagram above illustrates a sequence of events in granuloma formation in response to Mycobacterium tuberculosis (MTB). The key cell in the process is the epithelioid macrophage.

• Granulomatous diseases (mycobacteria, fungi)

• Tuberculin skin reactions

• Transplant rejection

• Contact dermatitis

Cytotoxic T lymphocyte (CTL) mediated responses: CD8+ T cells are generated and lyse specific cells. Class I HLA molecules play a role. Reactions with this mode include:

• Neoplastic cell lysis

• Transplant rejection

• Virus-infected cell lysis