Rh factor, ABO incompatibility

Discussion in 'Medical Students - MD' started by Gospe1, Mar 17, 2010.

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  1. Gospe1

    Gospe1 10+ Year Member

    Jan 1, 2007
    So Rh factor fetal-mother incompatibility makes sense. Why doesn't this happen with ABO? The mother can be OO and the baby be AO or BO. The mother would have anti-A and anti-B. Why no erythroblastosis fetalis? Wikipedia says something like most anti-A and anti-B are IgM which can't cross the placenta. Again, why are they igM when anti-Rh are IgG?
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  3. clinicallabguy

    clinicallabguy MS4 7+ Year Member

    Mar 30, 2008
    The reason is what you said. Anti-A and Anti-B are IgM antibodies, and cannot cross the placenta. They are IgM's because the epitopes they are specific for are carbohydrate moieties. So there is no T-cell help from CD4 cells, because there is no peptide presentation on MHC2. So no class switch.

    A antigen: N-acetylgalactosamine
    B antigen: Galactose
    On a precursor substance.

    The ABO blood group system is non-RBC immune, generally speaking. This means that anti-A and anti-B antibodies do not form as a result of a person being immunized with another persons blood. They form naturally, and the way I learned it is we do not know exactly why anti-A and anti-B antibodies form. They may actually be against antigens everyone sees, but cross react with blood group antigens is a hypothesis and negative selection explains why only the antibody specificity a person lacks is formed. But it's like clockwork that they form in everybody. If you're type A then you will have anti-B antibodies. It's amazing that it happens universally.

    The Rh system is RBC immune, and is a protein antigen. Protein->Peptide->MHC2->Th->Class switch to IgG. The Rh pos firstborn of an Rh neg mother will likely not develop hemolytic disease of the newborn, but the mother may form IgG anti-D antibodies after the insult with the babies blood. Then subsequent babies will be at risk if they are Rh pos.
  4. MicA


    Nov 12, 2009
    There are always preformed antibodies in your blood for the ABO system. Hence the immediate rejection of donor blood if it is not suitable. They are IgM. They cannot pass the placenta. I don't think that your body even class switches those antibodies to IgG otherwise it would affect the baby. We actually learnt that the AB antibodies could be due to your body developing something against the intestinal bacteria, though I don't think that people really know because you begin to develop antibodies at around 3 months. I asked this question to a lecturer, the one that you asked, and they mentioned IgM and the fact that sometimes the baby can be born with jaundice, especially if the mother is O and the baby is A or B.

    As you know anti-D is an IgG antibody so can pass through the placenta. Make sense because you need to be exposed first to blood at birth before you begin to develop the antibodies.
  5. deuce924

    deuce924 5+ Year Member

    Jun 10, 2007
    Just memorize that Anti-A and Anti-B are IgM which can't cross the placenta and Anti-D are IgG which can cross the placenta.

    I think the theory behind why ABO are IgM is because the commensal bacteria in your intestine possess these same antigens on the RBC's and are constantly being made to those antigens not found on the body's RBC's thus being IgM.

    Also, ABO Mismatch is more commmon than Rh Mismatch but it is rarely symptomatic.
  6. Ariodant

    Ariodant Fiat Lux 7+ Year Member

    Dec 12, 2007
    In most cases the ab will be IgM and blocked by the placenta. However, if the mother is type O, she will develop IgG against A and B, and her second incompatible baby will have hemolytic anemia of the newborn.
  7. Disinence2

    Disinence2 Emergency Medicine 7+ Year Member

    Aug 11, 2006
    Anyone Verify this?
  8. Jolie South

    Jolie South is invoking Domo. . . Moderator Emeritus 10+ Year Member

    We just had a lecture on this. ABO HDN exists and is actually becoming more common than Rh HDN (due to Rhogam prevention), but it's not as severe. Hydrops fetalis, kernicterus, and stillbirth are rare.
  9. dru2002

    dru2002 2+ Year Member

    Dec 23, 2008
    It's good to be an immunology major.
  10. Aphtalyfe

    Aphtalyfe 7+ Year Member

    Jul 28, 2009
    New York, NY
    Also, other tissues in the fetus, such as vWF, express A or B (depending on the blood type of the fetus) antigens and will effectively soak up some of the IgG that would've bound to the A or B on RBCs. However, Rh is only found on RBCs.
    Last edited: Mar 19, 2010
  11. JoshuaR

    JoshuaR Applying 2+ Year Member

    Aug 14, 2009
    As this is VERY related, I felt it more appropriate to respond to this thread than create another. This is merely for my own curiosity.

    Why DO Blood Type O mothers produce IgG antibodies against A or B? (we already saw in this thread that these are carbs, not peptides). And also, considering that Type O mothers can produce IgG, why DON'T A or B mothers produce IgG? Why do they only produce, for the most part, IgM?
  12. Leon22

    Leon22 2+ Year Member

    Jul 23, 2011
    anti-A anti B IgG do exist in group O individual(dont know how,but it is stated in goljan rr)
    but infant of blood group A/B/AB has poor ability of expressing the ABO antigen.
    But when it does happen it causes ABO incomptability and causes neonatal jaundice
  13. SpecterGT260

    SpecterGT260 Catdoucheus 2+ Year Member

    Feb 1, 2012
    If true I see no reason why a A mother couldn't form IgG against B and vice versa

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