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Type 3 hypersensitivity is confusing?

  1. Oct 23, 2011 #1
    Hello guys,

    I'm reading and trying to understand this and going no where. From what I gather in Type 3 hypersensitivity, there is too much antigen and small immune complexes are made, which are not phagocytosed and deposit in tissue causing tissue damage. Then what has Arthus reaction and serum sickness got to do with this concept (small immune complexes). I don't understand Arthus reaction and serum sickness, despite reading it for sometime now.
     
  2. jcsd
  3. Oct 23, 2011 #2
    These are simply examples of the type III reaction. What exactly are you unable to understand?
     
  4. Oct 23, 2011 #3

    bobze

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    T3H is when you have excess antigen (Ag) and too little antibody (Ab) being formed. This results in small immune complexes being formed which aren't phagocytized and potential deposition along vasculature or basement membranes.

    Arthus reaction is just an experimental way to demonstrate T3H. Its when you inject Ag into an individual with the specific Ab. At the injection site, there is excessive Ag which makes small immune complexes which deposit on the vasculature in the area leading to a red bump. Its not really relevant clinically.

    In serum sickness, you have a patient being given large amounts of preformed antibodies hoping to give them passive immunity against the insulting substance. Such as antivenom or antitoxin. The host can make an immune response to these Ab's, but since you are giving such large amounts (as was often the case in WWII for soldiers being given anti-tetanus toxin) there is excess antigen. This results in the deposition along the vasculature and BM of glomeruli (glomerulinephritis).

    The immune complexes cause local inflammation, histamine release, etc. That's why you get the red bump in Arthus, or the characteristic rashes of systemic lupus erythematosus.

    Hope that helps
     
  5. Oct 23, 2011 #4
    Thanks Bobze :smile: I understand serum sickness now. However arthus reaction, why are we giving excess antigen for no reason. Is it as you said just to demonstrate this effect. Also in Arthus reaction immune complexes seem to precipate unlike serum sickness where they are soluble. Why is that?
     
  6. Oct 23, 2011 #5

    bobze

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    Correct, Arthus is just an experimental way to demonstrate a T3H. Like I said, its not useful clinically. Historically it was used to demonstrate the type 3 reaction. Around the turn of the 19th century I believe.

    The immune complexes are still soluble. They are deposited in the small vasculature near the site of the injection, leading to vasculitis.

    In serum sickness, you're giving the antivenom/toxin via IV administration. This circumvents the small vasculature that would lead to an Arthus reaction. However, your blood still passes through small vasculature which is why the effects are disseminated and the effects extend to areas like the glomeruli.
     
  7. Oct 24, 2011 #6

    bobze

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    As an afterthought, The actual way you get vasculitis or vascular damage is because after deposition the Ag-Ab complex is capable of fixing complement.
     
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