Notes April 29

April 29, 2008
VI. Types of Hypersensitivity Reactions

a. Type I
*Allergies, Asthma
*IgE against common environmental antigen

Hygiene Hypothesis:
*Allergies are more prevalent in cleaner societies because don’t have parasites, so no target for IgE.

b. Type II (Cytotoxic Type)
*You make IgG or IgM against own antigens, kill own cells with CD8 T-cells.
-> Transplant rejection
-> Some prolonged bacterial infections -> antibodies against shared bacterial/ self epitopes
-> ex. Syphilis- T. pallidum bacteria shares a lots of epitopes with human cells; this will kick off the body's cytotoxic response

c. Type III (Immune-complex type)
*Make IgG or IgM against own free antigens (not bound to cell surface) clots of antibody and antigen. Block small vessels (like in the kidneys) and blockage -> symptoms
->ex. Lupus

d. Type IV (Cell-mediated type)
*Delayed-takes around 48 hours after exposure to have reaction
*This because reaction is against antigens from intracellular pathogen, that’s had its antigens presented to T-cells by APCs (in past) if re-exposed CD8+ T-memory cells recruited to site of re-exposure.
*The immune response increases every time of re-exposure, especially in some cases such as with the PPD test. This is why it's considered a hypersensitivity.

i. PPD test, and why we don’t vaccinate for tuberculosis in the US
*In United States rates are low and we have antibiotics that kill tuberculosis
*TB antigens injected under skin check around 48-72 hours later
*So we want a test that works
*If we vaccinated, PPD test wouldn’t accurately identify tuberculosis carriers
*Reverse of herd immunity in lots of other countries, lots of tuberculosis carriers, so vaccine makes more sense than the test

*If person has a positive PPD test, do not get the test again!!! could lead to severe complications, including death! (instead, will need chest x-ray)

TB vaccine: Live vaccine, related to tuberculosis bacteria
** CD8+ T-memory cells against vaccine bacteria
** Not a lot of good T-cell receptors antigens on tuberculosis or vaccine bacteria cell wall
** So T-memory cell response weak
** May only protect for about 15 years, if that.

*Tuberculosis stops phagolysosome formation
-> Survives in macrophages very well
-> Hard to make T-cell receptors antibodies against
-> So tuberculosis, like diamonds and herpes, is forever!

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