Delayed Hypersensitivity Reactions

Excerpt

Coombs and Gel classified type IV hypersensitivity reaction (HR) as a delayed hypersensitivity reaction (DHR), which takes more than 12 hours to develop. Typically the maximal reaction time occurs between 48 to 72 hours. Antibodies do not mediate DHR; it is mediated by T cells that cause an inflammatory reaction to either exogenous or autoantigens. This HR to exogenous antigens involves T cells and also antigen-presenting cells (APC) such as macrophages and dendritic cells, all produce cytokines that stimulate a local inflammatory response in a sensitized individual. The DHR to autoantigens can be seen in type 1 diabetes mellitus, which is an autoimmune disease that results from autoimmune cell-mediated destruction of insulin-secreting pancreatic beta cells. DHR cannot be transferred from an animal to another by means of antibodies or serum. However, it can be transferred by T cells, particularly CD4 Th1 cells, but it is progressively lost in individuals with HIV/AIDS. Antigen-presenting cells (APC) such as Langerhans cells engulf process and present antigens to antigen-specific T cells that become sensitized. Cytokines produced by keratinocytes, APC, and T cells recruit antigen-nonspecific T cells and macrophages to participate in a local inflammatory reaction.

There are three variants of delayed hypersensitivity as listed below and their maximal reaction time appears in brackets:

1. Contact (48 to 72 hours)

2. Tuberculin (48 to 72 hours)

3. Granulomatous (21 to 28 days)