Type IV hypersensitivity refers to a delayed hypersensitivity reaction that occurs as a result of cell-mediated immunity. It is an immune response mediated by T-lymphocytes, specifically CD4+ T cells, leading to inflammation and tissue damage. This type of hypersensitivity is known as delayed because it typically takes several hours to days to manifest after exposure to an antigen.
During a type IV hypersensitivity reaction, an antigen is encountered by antigen-presenting cells (APCs) such as macrophages. These APCs process and present the antigen to CD4+ T cells, known as T helper cells. The activated T helper cells release cytokines, which trigger an inflammatory response. Infiltration of macrophages and other immune cells, such as neutrophils, occurs at the site of exposure.
The tissue damage associated with type IV hypersensitivity is mainly caused by the release of toxic molecules and enzymes by the infiltrating immune cells. This immune-mediated response can lead to destruction of tissues and organs, contributing to the pathogenesis of various diseases, including contact dermatitis, granulomatous reactions, and certain autoimmune conditions.
Unlike immediate hypersensitivity reactions (type I, II, and III), which involve the production of antibodies, type IV hypersensitivity is antibody-independent. It is characterized by a delayed immune response, often resulting in sustained inflammation and tissue injury.
In summary, type IV hypersensitivities are delayed hypersensitivity reactions mediated by T lymphocytes, causing an inflammatory response that can lead to tissue damage. These reactions play a significant role in various immune-related diseases and are distinct from other types of hypersensitivity reactions that involve antibodies.
