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nbme24/Block 4/Question#4

A 63-year-old woman develops flank pain, ...

ABO incompatibility

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submitted by neonem(372),

This is acute hemolytic transfusion reaction, a type II hypersensitivity where pre-formed IgM antibodies bind to incompatible ABO antigens on donor RBCs, which causes intravascular hemolysis. Rh incompatibility, like colonelred_ said, comes more into play with Rh-compatibility of pregnancy and it is due to IgG antibodies, which more often cause extravascular hemolysis since splenic macrophages have those Fc-gamma-R receptors to bind whatever IgG has caught. Extravascular doesn't cause that hypotension, fever, flank pain associated with hemoglobinuria since the macrophages hold on to the degraded RBCs and convert it to biliverdin, which can safely be excreted by the liver.

mousie  Could you help me with understanding why this isn't a Type I HSR? I understand that ABO incompatibility is Type II HSR but I don't know how to tell the difference between a patient who is IgA deficient and having a Type I Reaction to an infusion vs ABO incompatibility .... +2  
sympathetikey  @mousie - https://imgur.com/QH5rCEX Basically, think of Type 1 HS like a normal allergic reaction (itchy, wheezing, etc.). Whereas, with ABO incompatibility you get the question's presentation. +2  
medpsychosis  When it comes to Acute hemolytic transfusion reactions, they are Type II hypersensitivity and divided into Intravascular (ABO) and Extravascular (host Ab against foreign antigen on donor RBC). The differentiating factor between them is simple. Intravascular (ABO) will present with hemoglobinuria alongside all the other common symptoms (fever,hypotension, tachypnea etc.) Extravascular hemolysis will stand out with Jaundice as one of the presenting symptoms. Hope this helps! +3  

Why was the acute hemolytic transfusion reaction due to ABO incompatibility, but not Rh incompatibility?

colonelred_  Rh incompatibility comes more into play with Rh- mother and Rh+ babies. +1  

 +0  upvote downvote
submitted by roygbiv(14),

Why could this not be extravascular hemolysis? In FA it says acute hemolytic transfusion reaction can be due to ABO incompatibility or extravascular hemolysis.

niboonsh  because extravascular hemolysis is associated w jaundice. Intravascular hemolysis would have hemoglobinuria but that's not an answer +  
niboonsh  i mean that is the answer lol +  
krewfoo99  According to pathoma: Intrvascular haemolysis will lead to haemoglobin binding to haptoglobin. This complex will travel to the kidneys and be excreted. This will lead to red colored urine and haemosiduria (Note: This can also lead to acute tubular necrosis) Extravascular haemolysis is when macrophages break down the RBC. Then the Haeme is converted to biliverdin then bilirubin and conjugated in liver, and then excreted. +  

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submitted by haliburton(141),

FA 2017 states that extravascular hemolysis has jaundice where ABO incompatibility would not.

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submitted by lilmonkey(14),

To conclude: 1) ABO: IgM -> Complement -> Intravascular 2) Rh: IgG -> Splenic Macrophage -> Extravascular