Mechanisms of Hypersensitivity Reactions

Type IV Hypersensitivity: Delayed-Type Cell-Mediated Reactions

Type IV hypersensitivity is a cell-mediated immune response that occurs over 24 to 72 hours after exposure to an antigen. Unlike other hypersensitivity types, it does not involve antibodies but is driven by T-cell activation.

Mechanism

Upon exposure to a specific antigen, sensitized CD4+ T cells release cytokines, leading to macrophage activation and inflammation. This reaction can result in tissue damage.

Clinical Manifestations

• Contact dermatitis
• Tuberculin skin test reaction
• Granulomatous inflammation

Examples and Diagnosis

Type IV hypersensitivity can be diagnosed through delayed skin tests such as the Mantoux test for tuberculosis.

Treatment

Management involves avoiding the causative antigen and using corticosteroids or immunosuppressants.