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If rhogam is IgG why doesn't it cross the placenta??

Discussion in 'Step I' started by Taus, Jun 15, 2007.

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  1. Taus

    Taus . Moderator

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    I can't think of a logical reason why this IgG wouldn't just go kick the babies a$$.... I remember goljan saying something about it but don't have time right now to re-listen to it
  2. cdql

    cdql

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    I could be wrong but I thought that Rhogam did cross the barrier. It's just meant to be a blocking antibody so it binds to the Rh+ cells and does nothing. (well, by nothing, I mean it prevents the mother from creating antibodies)

    Whereas the anti-Rh antibody from the mother (also IgG) will cross the placenta and cause an immune reaction.
  3. lord_jeebus

    lord_jeebus 和魂洋才 Moderator

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    I'm pretty sure Rhogam doesn't cross the placenta, but I don't know why.
  4. SOUNDMAN

    SOUNDMAN Senior Member

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    In his RR pg. 282.. B.2.b - Anti-D globulin does NOT cross the placenta. Lasts 3 months, given at 28 weeks and after delivery. Also in his HY blue notes: "Special tests are performed on the mother's blood that detect fetal RBC's in her blood. The amount of fetal blood is quantified so that the appropriate amount of anti-D globulin is given to the mother....so that the mother does not host an antibody response against the D antigen."

    Hope that helps.
  5. somnolent

    somnolent Member

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    I actually had a UW question about this today. Basically they said that IgG is used b/c the Fc receptors on the phagocytes have a greater affinity for the Fc fragments on IgG than the other isotypes. So while this told me that IgG is used because it was the "best" for fetal antigens, it didn't explain why it doesn't just cross over into the baby.

    So, I checked with goljan. He was pretty vague. He said that anti-D comes from sensitized mothers, and it's been heat treated, and stuff. So I guess it's one of those "just because" answers. I can't find a better answer anywhere else. My guess is that it's packaged in a way so that it doesn't.
  6. automan2

    automan2

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    Some kid actually just asked our professor this, and rhogam does cross the placenta. Think about it. It's IgG, and we have been told for our whole lives the IgG crosses. Why would this one be any different. Goljan is the man, but he explains concepts so we understand them, he doesn't always use scientific fact.


    Clinnically, it isn't used very often during pregnancy anymore. And those that do get it, typically are given a does that quickly bind the fetal cells before crossing the placenta. (Yes, I know this is a bad theory, and my profesor doesn't agree with it either, but that is what is being used to make clinical decisions)

    The majority of the time rhogam is used, is post partum so there is no worry about it crossing the placenta.

    Bottom line: yes it does cross the placenta, but it is dosed to have minimal effect.
  7. Miami_med

    Miami_med Moving Far Away Moderator Emeritus

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    I promise that it's still used during pregnancy.
  8. Army DOC DO

    Army DOC DO Anesthesia attending

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    Ah your professor is incorrect, it does not cross the placenta. an IGG does but Rhogam is an Anti-D globulin, it comes from other women who have been sensitized and it is heat treated and all that stuff, so it does not cross. if it crossed the placenta it would not do its job, it would cause a hemolytic anemia in the baby if it crossed over. not doing its job. Its a prophylaxis. The idea of Anti-D (rhogam is to prevent the mothers sensitization to the fetal/maternal mixing of blood. which can occur pre delivery and of course at delivery. Anti Globulin D jumps onto the fetal RH+ blood if the mother gets exposed to it and masks the binding Sites or Destroys the fetal RBC so the mother does not mount an antibody response. it does ot work if the mother has already had a Rh+ pregnancy and she is RH- she is sensitized and the Rhogam will do nothing. and the event of another pregnancy could result in kernicterus, a combined left and right sided heart failure, leading to hydrops fetalis etc. because the anti D IGG crosses over and attached to baby RBC, and splenic macrophages cause a hemolytic anemia.
    now one protecting thing is for a ABO type O mother is that they have the IGM circulating anti-A or Anti-B. so if she has a ABO baby that is type A, even if a fetal RBC gets into circulation that RBC will last a mili-second and be destroyed and thus protect the mother from mounting an anti-D antibody. but Rhrogam is given just the same because nothing is perfect. :) Happy Nerding all. :D
  9. automan2

    automan2

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    Maybe you should go back to school........... I think you are getting Anti-D and IgG confused. They call it RhoGam for a reason. Its short name is RhIG, an IgG antibody against Rh. It crosses the placenta. Period.

    Straight from the Rhogam Literature.....

    "Use in pregnancy. RhIG is administered to Rh negative women who are pregnant (or recently delivered) and do not already have alloimmunization to the D antigen. Routinely one vial is administered at 28 weeks of gestation and a second within 72 hours following delivery. These time recommendations are somewhat arbitrary. RhIG can and should be administered even if 4 or 5 days have elapsed since delivery. No one is really sure when the time cut-off for effectiveness is, so it's best to go ahead and give it just to be sure. Additional vials of RhIG may be indicated at the time of delivery if a larger-than-normal fetomaternal hemorrhage occurred. See the section on fetomaternal hemorrhage detection for further details.
    RhIG is also indicated at the time of ectopic pregnancy, amniocentesis, abortion or miscarriage, or abdominal trauma in Rh negative, unimmunized women.
    RhIG is IgG antibody. It will cross the placenta and can cause a weakly positive DAT in the fetus/infant. However, the titer of anti-D is too weak to cause any clinically significant RBC destruction. An Rh negative woman administered RhIG will generally develop a detectable anti-D with a titer in the range of 1 to 4. Occasional women sampled right after RhIG administration may have a titer of up to 8. Any higher titer of anti-D should be considered suspicious of true alloimmunization. However, if there is doubt as to whether an anti-D detected prenatally represents true alloimmunization or not, further RhIG probably should be administered at the time of delivery to make sure.
    The anti-D of RhIG can be detected in some women for 3 to 5 months after administration of RhIG.
  10. Army DOC DO

    Army DOC DO Anesthesia attending

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    hey I am going off of my pharm/ OB lectures and and the Goljan Path book and his Audio lecture for Boards, yes it is an IgG, however, it comes from sensitized mothers, and is heat treat and various other things that makes it unable to cross the placenta. I have 4 different sources that says it doesn't sorry. Just going on what I have. :)
  11. Pinkertinkle

    Pinkertinkle 2003 Member

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    Sounds like something a little too controversial for the boards to actually test.
  12. automan2

    automan2

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    agreed! Wise words from pink
  13. Army DOC DO

    Army DOC DO Anesthesia attending

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    yeah Goljan, states in his laudio lecture that they just want you to know that it covers the RBC of the Rh+ fetal blood if it gets into the mothers circulation to "hide the receptors" from the the mothers immune system so she never mounts a response to it. :)
  14. Freshnstylin

    Freshnstylin Member

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    Not to put a kink into your debate but I had one question on Rhogam on the USMLE (took it last week). It was confusing so I picked and clicked.
  15. NEATOMD

    NEATOMD Senior Member

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    I think the point is that they aren't going to ask...
    "Does Rhogam cross the placenta?"
    a.) yes
    b.) no
    c.) I'm not sure because my mom says it does but I read a Goljan lecture that said it didn't.
    d.) "I was just talking about Shaft."
  16. Snelgrave

    Snelgrave Snelgrave

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    yeah i thought this was interesting when i read it in UW, but I imagine this detailed discussion is low yield.
  17. Twptophan

    Twptophan

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    It has to do with the dose, and how much fetal blood actually gets into the mom. In a typical pregnancy, only a few ml of fetal blood gets into the mother. If she is RH -, then she has no D antigen anywhere in her body, and thus only a very small about of the anti-D (i.e., rhogam) is needed to prevent sensitization in the mom. Think of the amount of a usual maternal total body blood volume (about 6-7 liters) and if only a few ml of fetal blood gets into mom, then not much rhogam is needed to soak up these few fetal D antigens.

    Yes, the rhogam does cross the placenta, and but so little is used in the mom, that the amount that gets into baby is not enough to cause a clinically significant fetal hemolysis.

    If there is a more significant fetal-maternal hemorrhage, (like with placental abruption or trauma) then they can do maternal D titers and figure out exactly how much rhogam is needed, there are formulas for this.
  18. NRAI2001

    NRAI2001 3K Member

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    Awesome explanation...even though whether or not Rhogam crosses the placenta is still controversial (I think the consensus here on SDN is leaning towards it crossing). You made a few really good points:

    1) Rhogam basically does the mothers immune systems job of picking off the Rh+ RBCs... hence the mother never developes the IgGs.

    2) If Rhogam were to cross the placenta (even if engineered not to do so, some would still invariable cross) it would cause kernicturus in the fetus.. the result we are specifically attempting to avoid..

    3) I also read in Goljan how O- mothers have the added benefit of having preformed anti A and anti B Abs against fetal RBCs, allowing for rapid removal of any fetal blooding mixing before IgG can be formed..

    4) So the source of Rhogam is from other sensitized women?? Sorry my pharm is a bit rusty. How does heat treating it effect it? Does it prevent a maternal reaction against the rhogam?
  19. buckeye12

    buckeye12

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    So does Rhogam target Rh+ RBCs for destruction before an immune response can be formed, or does it just hide the D antigen by competitively and irreversibly blocking that antigen?
  20. turkeyjerky

    turkeyjerky

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    as much as I hate to think a pre-med is gonna come in here and school a bunch of med students and residents, this makes sense to me...
  21. NRAI2001

    NRAI2001 3K Member

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    Competitive and irreversibly blocking? I think those terms are used more for receptor kinetics... IgG binds the Rh+ RBCs and leads to splenic removal. You were correct in that the goal of this is to quickly do this before the mothers immune system can notice and mount its own attack..
  22. NRAI2001

    NRAI2001 3K Member

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    "Routinely one vial is administered at 28 weeks of gestation and a second within 72 hours following delivery.."

    Why is it that only one dose is needed for mothers? Does the rhogam remain in the mothers system for the next couple of weeks? If it does remain for so long how does it not eventually cause jaundice in the fetus (even if it only enters at low levels)??
  23. Twptophan

    Twptophan

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    IgG has a half life of 21 days. I'm not sure what the half life of Rhogam is though, but Rhogam is an IgG.
    There will be some hemolysis in the fetus, though will not be significant (due to the low dose). It would probably be hard to distinguish it from physiological jaundice of the newborn which is due to a short-lived lacking of bilirubin conjugation enzymes.

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