DTH cells are T cells that are recruited into tissues and activated by antigen-presenting cells to produce cytokines that mediate local inflammation in delayed-type hypersensitivity (DTH) reactions.
Understanding Delayed-Type Hypersensitivity (DTH)
Delayed-type hypersensitivity (DTH) is a type of cell-mediated immunity characterized by a delayed inflammatory reaction to a foreign antigen. This reaction typically peaks 48-72 hours after antigen exposure. DTH is crucial in defending against intracellular pathogens and also plays a role in various pathological conditions.
The Role of T Cells in DTH
T cells are central to DTH reactions. Traditionally, it was thought that CD4+ T cells were the primary mediators. However, research has revealed that CD8+ T cells also participate in DTH responses.
- CD4+ T Cells: These cells recognize antigens presented by antigen-presenting cells (APCs) in the context of MHC class II molecules. Upon activation, they release cytokines like interferon-gamma (IFN-γ), which activates macrophages and promotes inflammation.
- CD8+ T Cells: These cells recognize antigens presented by APCs in the context of MHC class I molecules. They can also release cytokines and directly kill target cells. CD8+ T cells are known to mediate DTH responses in allergic contact dermatitis, drug eruptions, asthma, and autoimmune diseases.
Mechanism of Action
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Sensitization Phase: Initial exposure to an antigen leads to the activation and proliferation of antigen-specific T cells.
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Elicitation Phase: Subsequent exposure to the same antigen triggers the recruitment of these sensitized T cells to the site of antigen exposure.
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Inflammatory Response: At the site, T cells are activated by APCs and release cytokines, leading to local inflammation. This involves:
- Cytokine Production: IFN-γ activates macrophages, enhancing their phagocytic and antigen-presenting capabilities.
- Cell Recruitment: Chemokines attract more immune cells, amplifying the inflammatory response.
- Tissue Damage: Prolonged inflammation can cause tissue damage and contribute to disease pathology.
Examples of DTH Reactions
- Allergic Contact Dermatitis: Skin inflammation caused by contact with allergens like poison ivy or nickel.
- Drug Eruptions: Skin reactions resulting from drug exposure.
- Tuberculin Skin Test: A diagnostic test for tuberculosis where a positive reaction indicates prior exposure to Mycobacterium tuberculosis.
- Autoimmune Diseases: DTH responses can contribute to tissue damage in diseases like rheumatoid arthritis and multiple sclerosis.
- Asthma: Chronic airway inflammation where DTH responses may play a role.
Table Summarizing DTH Cells and Their Roles
| Cell Type | Role in DTH |
|---|---|
| CD4+ T Cells | Recognize antigens via MHC Class II, release cytokines (e.g., IFN-γ) to activate macrophages and promote inflammation. |
| CD8+ T Cells | Recognize antigens via MHC Class I, release cytokines, and can directly kill target cells. Important in allergic contact dermatitis, drug eruptions, etc. |
