Type IV Hypersensitivity Reaction
These reactions are due to cell mediated immune responses mediated by T helper lymphocytes (Th) and T Cytotoxic lymphocytes (Tc).
T helper lymphocytes; activate B cell to secrete antibodies, macrophages to destroy ingested microbes, help to activate Tc.
T cytotoxic lymphocytes (Tc); kill infected target cells
When Th and Tc come into contact with antigen, they release cytokines (stimulating immune system to fight foreign attack) that signal to other blood cells, including macrophages.
Macrophages increase in number in affected tissues and cause local damage.
Two Phase of Type IV Hypersensitivity
1. Sensitization phase
When antigen interact cells to Th cells, this lead to production of more more Th cell specific to antigen.
2. Effector phase
In effector phase, antigen react with specific Th lymphocytes which release cytokines.
Cytokines act on macrophages to activate them to produce toxic oxygen molecules, cause local tissue damage.
Two forms of Type IV Hypersensitivity
1. Tuberculin reaction
When a small dose of tuberculin is injected into the skin of sensitize individual (prior infection or immunization), a raised inflammatory develops at the site within 48-72 hours.
Tuberculin test provides a useful indication of state of delayed hypersensitivity to bacilli.
2. Contact dermatitis
Delayer hypersensitivity result from skin contact with chemical or metal such as; nickel, chromium, drug (penicillin) and soap, shampoo and cream etc.
Cell in skin take up these chemical and represent them to T lymphocytes, which react with antigen, causing release of cytokines and activate other immune cells, which result damage to tissue and lesion.
Sensitivity is indicated by itching in 4-5 hours and local reaction occurs after 24-48 hours.
It can be detected by Patch test.
Tuberculin Type of Hypersensitivity (Delayed Type Reaction)
1. Tuberculin test (mantoux test)
Skin test to detected if have infected TB bacteria.
2. Frei test
Skin test to identify lymphogranuloma venereum LGV), sexually transmitted bacterial infection causes by gram -ve bacteria, Chlamydia trachomatis.
3. Mumps and Herpes Simplex test
4. Casoni skin test
Hydatid disease, infection with dog tapeworm; egg in feces
Affect liver, lungs, brain.
5. Ducrey tests
6. Candida skin test
7. Filariasis skin test
8. Brucellergen test
9. Histoplasmin test
10. Blastomycin test
11. Toxoplasma skin test
12. Patch test; Allergic skin test
Diagnostic Evaluation
Diagnostic Hypersensitivity Skin Tests are two types;
1. Anaphylatic type of hypersensitivity
(Immediate reaction)
2. Tuberculin Type of Hypersensitivity
(Delayed type reactions)
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