Hypersensitvity Type 4 Flashcards
What are hypersensitivity reactions that involve antibodies called?
Humoral-mediated responses
What cells does hypersensitivity type 4 mainly involve?
It’s a (T) cell mediated response
Involves T helper cells (CD4+ cells) and cytotoxic T cells (CD8+ cells)
What are examples of hypersensitivity type 4 reactions?
Poison Ivy
Tuberculosis (TB) skin test
Contact dermatitis (resulting from metal allergies, or exposure to latex)
How does poison ivy cause a reaction?
Contains urushiol > alters skin proteins (they’re now immunogens) > body now sees proteins as foreign > APC recognises and process immunogens > fragment of immunogen on MHC class 2 protein on APC surface presented to Th cell > Th cell binds > APC releases cytokines to help Th cell differenciate > tissue damage > vasicular dermatitis (rash)
What happens if a naive Th cell is stimulated by cytokine IL-12?
Becomes a Th1 cell
What is the role of Th1 in type 4 hypersensitivity?
Th1 releases cytokines (including IFN-gamma)
IFN-gamma activates macrophages > release TNF-alpha + IL-1 > increases receptors on endothelial cells that recruit leukocytes
Active macrophages perform phagocytosis and release reactive oxygen species + lysosomal enzymes > causes tissue damage
What happens if a naive Th is stimulated by cytokine IL-6?
Becomes a Th17 cell
What is the role of Th17 in type 4 hypersensitivity?
Th17 releases IL-17
IL-17 activates neutrophils > causes similar damage as macrophages
Why does type 4 hypersensitivity take longer to react than the other types?
Is cell mediated
Takes a while before T cells are activated and recruited, and have responded
Can take 24-72 hours for a reaction to occur
What is type 4 hypersensitivity also referred to as?
Delayed Type Hypersensitivity (DTH)
How does a Tuberculosis (TB) skin test cause a reaction?
Test determines if a person has been infected with mycobacterium tuberculosis
Protein components of bacteria are injected into the skin > if person has previously been infected, immune system attacks injected proteins through type 4
Positive test shows raised lesion at site of injection within hours-days
How does cytotoxic T cell mediated Type 4 Hypersensitivity work? Give examples
Cytotoxic T cells recognise and attack target cells directly
Tc cells release a vesicle containing a perforin-granzyme complex
Target cell takes in vesicle through endocytosis
Perforins make pores in vesicle > release granzymes (proteases) > induce apoptosis
Tc cell may also bind to a FAS receptor on target cell using FAS ligand > induces apoptosis
Type 1 diabetes:
Tc cells attack insulin secreting beta cells of pancreas
Rheumatoid Arthritis:
Tc cells attack joint tissue
Hashimoto’s disease:
Tc cells attack thyroid hormone producing follicular cells
Graft rejection:
Tc cells attack foreign grafted cells
