Hypersensitivity type 4 Flashcards
What cells cause hypersensitivity type 4 reactions
T cells and macrophages
NO antibody is involved
The lesion that forms in type 4 hypersensitivities is called a
Granuloma
Why are type 4 hypersensitivities called delayed hypersensitivity
Takes at least 12 hours to develop
Response lasts for days
What is the major veterinary application for type 4 hypersensitivities
Testing for mycobacteria disease using intradermal (ID) skin tests
True or false
Skin tests can test for current OR previous infections with intracellular bacteria
True
Particularly mycobacterial infections
What is the mechanism for testing with ID skin tests, how long should you wait for swelling to occur?
Inject a purified, killer cell extract intradermally
use tuberculin to test for TB, Johnin for bovine paratuberculosis
Memory T cells rapidly respond to mycobacterial proteins in infected individuals
Observe for swelling 24-96 hours later
Describe the process in a positive reaction to an ID skin test
Antigen is phagocytosed by dendritic cells (present on their surface) (also called langerhans cells in the skin)
These dendritic cells move to lymph nodes And activate memory T cells
The T cells divide and migrate back to the site of the antigen and attract more lymphocytes, macrophages and basophils
See granulomas (swelling of the skin) in areas where the cell extract was injected
Why do false positives to ID skin tests occur?
Due to cross reaction with similar proteins from other mycobacteria
Describe a negative reaction to an ID skin test
No previous or current infections
No memory T cells
Purified extract does not elicit an immune response within 72 hours
What are 2 common disease that involve delayed hypersensitivities
Mycobacterial diseases (bovine TB also called Wood Bison, or Johnei’s disease)
Allergic contact dermatitis
Describe the mechanism behind mycobacterial diseases
Microbes live intracellularly in macrophages (protected by a waxy coat)
Controlled by cell mediated immunity:
- cytotoxic T cells and macrophages
- T cells secrete mediators that attract macrophages and make them more effective at killing microbes (only partially effective)
- large granulomas called tubercles often develop (one of the necropsy signs of tuberculosis)
What is allergic contact dermatitis
Some highly reactive chemicals bind to skin proteins (formaldehyde, poison ivy, insecticides in flea collars)
Can provoke cell mediated immune response and delayed hypersensitivities
Signs: inflammation, severe pruritis, possibly skin vesicles (granulomas)
