So I think I understand the basic pathophys of the Rh+ fetus and Rh- mother and subsequent pregnancies...
The mother has the chance to develop antibodies (IgG?) to the Rh on fetal RBC's, which if she became pregnant again with another Rh+ fetus it could prove fatal (fetal erthyrocyte hemolysis due to mother's IgG Rh antibodies).
What I don't understand... Why wouldn't the administration of IgG anti-Rh (RhoGam) to a pregnant mother harm the fetus by the same mechanism as above? Is it just because the amount of drug administered is much less than in an actual immune b cell response? The RhoGam binds to fetal Rh+ in the mother's circulation, correct?
Anybody know?
Hello, I am a baby red cell. On me is an antigen. Its called Rh+.
Hi, I am mom's immune system. I don't know what this Rh+ is all about. I've never seen it before, I don't like it, and so, I think its foreign. MOUNT UP! (
This is mom's immune system recognizing that a foreign antigen has entered her system, and forms IgM and then eventually IgG with memory cells against that antigen, much like any antigen would do)
Hello, I am a baby red cell from baby #2. On me is an antigen. Its called Rh+.
Hi, I am moms... THAT LITTLE F-UCKER! HE'S BACK AGAIN!? KILL THAT MOTHER F-UCKER! I TOLD YOU... NOT IN MY HOUSE! (
Mom's immune system, having already been sensitized mounts an immediate IgG response against baby's red blood cells, having recognized this not just as foreign, but as an already-encountered known "bad guy")
Hi, I'm RhoGam. I'm sort of like Frodo's elven cloak of invisibility. For those of you a little bit older, "Ninja!" (smoke grenade) "Vanish!" (
RhoGam has the Fab portion of the antibody against Rh+... you know, the V part of the Y... so it binds to Rh+ really well. What it lacks is the Fc component... you know the | part of the Y... so it doesn't do anything once its bound.
Hi, I a moms immune system. I could have SWORN there was that Rh+ thing back here again. Now where did he go... Well, I dont see that antigen anymore, there's nothing for my Fab to bind to. And, I don't see any signal from my Fc communicator, so none of my Fab's have bound to anything. Oh well, false alarm.
So you see, it has nothing to do with dosage of antibody/immunoglobulin, what it has to do is with the functionality of the antibodies doing the binding. IgM cannot cross the placenta, so the initial baby is unharmed. IgG CAN cross the placenta, and has Specificity to identify the Rh-Anti-D Antigen via its Fab portion. The normal IgG Fc portion summons the rest of the immune response, mostly via complement activation, but also by identification to macrophages to consume and destroy the red cell. This IgG from Mom, the one with a functional Fab and Fc portion, that CAN cross the placenta, marks all of the baby's red blood cells for death.
Inject RhoGam-D. This is a rhesus antibody that matches mom's antibodies in almost every way. IgG, so it crosses the placenta. Specific for Rh-Anti-D Antigen via Fab portion, so it binds to baby's red blood cells. BUT, it has a nonfunctioning Fc portion (mom's immune system doesn't know what to do with Rhesus Fc) so no immune response is summoned. So, the antigen is protected ("hidden") by the binding of antibody, but the cell is not destroyed since the Fc portion doesn't work (thus Frodo's cloak of invisibility).
WINN!
