Delayed-type hypersensitivity response

Delayed-type Hypersensitivity Response

The delayed-type hypersensitivity (DTH) responseis a form of cell-mediated immune reaction that occurs later than the immediate hypersensitivity reactions. It is primarily mediated by T cells rather than antibodies and is a crucial component of the immune system's ability to respond to certain pathogens, particularly intracellular pathogens such as mycobacteria, fungi, and some parasites.

Mechanism

The DTH response is initiated when antigen-presenting cells, such as macrophages or dendritic cells, present antigens to T helper cells (specifically, CD4+ T cells). Upon recognition of the antigen, these T cells become activated and secrete cytokines, such as interferon gamma (IFN-γ), which further activate macrophages and other immune cells.

The hallmark of the DTH response is the recruitment and activation of macrophages at the site of antigen exposure. This leads to inflammation and tissue damage, which is why DTH responses are often associated with conditions such as contact dermatitis and the tuberculin skin test.

Clinical Significance

The DTH response is utilized in clinical settings to test for prior exposure to certain antigens. The most well-known example is the tuberculin skin test, also known as the Mantoux test, which is used to determine if a person has been exposed to Mycobacterium tuberculosis. In this test, a small amount of tuberculin is injected intradermally, and the site is observed for induration and erythema after 48-72 hours.

DTH responses are also involved in the pathogenesis of certain autoimmune diseases, such as type 1 diabetes and multiple sclerosis, where the immune system mistakenly targets self-antigens, leading to chronic inflammation and tissue damage.

Examples

- Contact Dermatitis: A common example of a DTH response, where skin contact with certain allergens, such as poison ivy or nickel, leads to an inflammatory skin reaction. - Tuberculin Skin Test: A diagnostic tool for tuberculosis exposure, relying on the DTH response to tuberculin. - Granuloma Formation: In chronic infections like tuberculosis, a DTH response can lead to the formation of granulomas, which are organized collections of macrophages and other immune cells.

Also see

• Hypersensitivity
• Cell-mediated immunity
• T helper cells
• Cytokines
• Autoimmune diseases