T cell mediated hypersensitivity reaction Robbins based md pathology exam answer easy to understand and replicate
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Robbins T-cell mediated hypersensitivity type IV mechanisms examples delayed-type hypersensitivity cytotoxic T cell
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T Cell-Mediated Hypersensitivity Reaction
Type IV Hypersensitivity / Delayed-Type Hypersensitivity
Definition:
Type IV hypersensitivity is a T lymphocyte-mediated immune reaction that causes tissue injury. It is not mediated by antibodies. It usually appears after 24-72 hours, hence it is called delayed-type hypersensitivity (DTH).
Type IV hypersensitivity is a T lymphocyte-mediated immune reaction that causes tissue injury. It is not mediated by antibodies. It usually appears after 24-72 hours, hence it is called delayed-type hypersensitivity (DTH).
The reaction is produced by:
- CD4+ helper T cells causing cytokine-mediated inflammation
- CD8+ cytotoxic T cells directly killing target cells
Robbins describes these as the two main mechanisms of Type IV injury: cytokine-mediated inflammation by Th1/Th17 cells and direct cytotoxicity by CD8+ T cells. A standard overview also describes Type IV reactions as delayed, antigen-specific T-cell responses. NCBI review
Mechanism of Type IV Hypersensitivity
1. Sensitization phase
- On first exposure, an antigen is taken up by antigen-presenting cells (APCs), especially dendritic cells/macrophages.
- APCs present antigen to naïve T lymphocytes.
- T cells become sensitized and differentiate into:
- Th1 cells
- Th17 cells
- CD8+ cytotoxic T lymphocytes
Usually, no significant tissue damage occurs during this first exposure.
2. Effector phase
On re-exposure to the same antigen, sensitized T cells react and produce tissue injury by two pathways.
A. Cytokine-Mediated Inflammation
This is mediated mainly by CD4+ Th1 and Th17 cells.
Th1 pathway
- Th1 cells secrete IFN-gamma.
- IFN-gamma activates macrophages.
- Activated macrophages release inflammatory mediators, enzymes, reactive oxygen species, and cytokines.
- These substances cause tissue injury and inflammation.
Th17 pathway
- Th17 cells release IL-17 and other cytokines.
- These recruit neutrophils and other leukocytes to the site.
- The recruited cells amplify inflammation and tissue destruction.
Result
- Local redness, swelling, induration, and mononuclear cell infiltration occur.
- Persistent Th1-mediated macrophage activation may produce granulomas.
Flowchart to reproduce in exams:
Antigen → APC → CD4+ T cells → Th1/Th17 cells → cytokinesTh1 → IFN-gamma → macrophage activation → tissue damageTh17 → IL-17 → leukocyte recruitment → inflammation and tissue damageB. T Cell-Mediated Cytotoxicity
This is mediated by CD8+ cytotoxic T lymphocytes (CTLs).
- CD8+ T cells recognize antigen expressed on target cells along with MHC class I molecules.
- CTLs directly kill these cells by:
- Perforin-granzyme pathway
- Fas-Fas ligand-mediated apoptosis
- CD8+ T cells may also release IFN-gamma, increasing inflammation.
Flowchart:
Antigen-bearing cell → recognition by CD8+ CTL → direct cell killing → tissue injuryExamples of Type IV Hypersensitivity
| Mechanism | Examples |
|---|---|
| Delayed-type inflammation | Tuberculin skin test, contact dermatitis, granulomatous inflammation in tuberculosis |
| Th1-mediated macrophage activation | Tuberculosis, leprosy, some fungal infections |
| CD8+ T-cell cytotoxicity | Type 1 diabetes mellitus, viral hepatitis, graft rejection, some drug reactions |
| Both CD4+ and CD8+ T cells | Multiple sclerosis, rheumatoid arthritis, inflammatory bowel disease |
Important Examples
1. Tuberculin (Mantoux) skin test
- A person previously sensitized to Mycobacterium tuberculosis receives intradermal tuberculin.
- Memory Th1 cells recognize the antigen.
- Cytokines recruit and activate macrophages.
- After 48-72 hours, there is induration at the injection site.
2. Contact dermatitis
Examples: poison ivy, nickel, cosmetics, rubber, dyes.
- Small chemicals act as haptens and bind host proteins.
- These altered proteins are recognized by T cells.
- On repeated contact, T cells cause dermatitis with erythema, vesicles, and itching.
3. Granulomatous inflammation
- Persistent intracellular microbes, such as Mycobacterium tuberculosis, stimulate prolonged Th1 responses.
- IFN-gamma continuously activates macrophages.
- Activated macrophages transform into epithelioid cells and form granulomas.
4. Type 1 diabetes mellitus
- Autoreactive CD8+ T cells destroy pancreatic beta cells.
- This leads to insulin deficiency.
Difference from Types I, II, and III
| Feature | Type IV hypersensitivity |
|---|---|
| Main mediator | T lymphocytes |
| Antibody involvement | Absent |
| Onset | Delayed, usually 24-72 hours |
| Main cells | CD4+ Th1/Th17 cells, macrophages, CD8+ CTLs |
| Main injury | Cytokine-mediated inflammation and direct cell killing |
| Examples | Mantoux test, contact dermatitis, TB granuloma, type 1 DM |
Short Exam Answer
Type IV hypersensitivity is a delayed, T-cell-mediated immune reaction that occurs 24-72 hours after exposure to antigen. It is not mediated by antibodies. In the sensitization phase, antigen is presented by APCs to T cells, leading to formation of Th1, Th17, and CD8+ effector cells. On re-exposure, Th1 cells secrete IFN-gamma and activate macrophages, while Th17 cells produce IL-17 and recruit leukocytes. This causes cytokine-mediated inflammation and tissue injury. CD8+ cytotoxic T cells can directly kill antigen-bearing target cells through perforin-granzyme and Fas-FasL pathways. Persistent Th1 activation may cause granuloma formation. Examples include tuberculin reaction, contact dermatitis, tuberculosis, type 1 diabetes mellitus, and transplant rejection.
Memory line
“Type IV = T cells, takes time, causes tissue damage.”
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