Here is some food for thought.
Think about which of the following scenario is worse:
1- Mom is O- and baby is O+ first pregnancy
2- Mom is O- and baby is O+ second pregnancy
3- Mom is O+ and baby is O+ second pregnancy
4- Mom is O+ and baby is A+ second pregnancy
5- Mom is O- and baby is A+ second pregnancy
6- Mom is O+ and baby is O- first pregnancy
Answer is 2
Rh incompatibility in second pregnancy. In presence of ABO incompatibility, Rh incompatibility, has lesser effect.
The most common group O has anti A IgM, anti B IgM and anti AB IgG.
Group A has anti B IgM.
Group B has anti A IgM.
Group AB has no antibodies.
So if I was dumb enough to transfuse GroupyA blood to a group B guy there would be hemolysis. But what would be the mechanism for this?
The patient with group B would have anti A IgM. IgM is a very potent complement activator. IgM is very trigger itchy, it first shoots the cell and then asks questions. So this hemolysis is very fast.
Now coming to the Rh question, imagine there was a mom with O- group and baby with O+ group.
In first pregnancy, the mom is not exposed to the Rh antigen until delivery, so the 1st baby is safe. But there would be a mixture of baby and mom's blood.
Now imagine a weird person (Rh+ cell) walking through an airport, he would taken by the TSA (macrophage) for an "interrogation". So the macrophages do this interrogation (phagocytosis) in the dark corners of spleen and pick up info (antigens) about these weirdos. This info is passed to T cells and they issue warrants to B cells (IgG) for identifications of these guys and they are killed on site (IgG mediated destruction)
You can see that this will, obviously, take time time. When she gets pregnant with Rh incompatibile kid again, the IgG have been synthesized and they cross placenta and attack the baby RBC's. Voila - Hydrops fetalis.
Now imagine a mom who is O- and has a baby with A+ group. This time, at delivery, there is mixing of blood again!
But the mom has anti A IgM which is so fast like a ninja, kills of the majority of the cells before they go for their interrogation with macrophages in spleen... So ABO incompatibility actually protects against the Rh sensitization.
What's the clinical significance of Rh incompatibility?
Whenever you take care of a pregnant lady, you will confirm her blood group and if you suspect Rh incompatibility you would give her "Rh IgG" (standard dose) at 28 weeks, even though the fetal blood is not exposed to mom's immune system, this is done just in case - there might be a fall, injury etc and baby's blood may get into mom's circulation.
Why do you give Rh IgG when you want prevent the disease which is itself caused by IgG?
Rh IgG are heat treated and they cannot cross the placental barrier unlike normal IgG.
And finally, you give another dose of Rh IgG after delivery. But this time, you actually estimate the amount of fetal blood which is mixed with mom's blood by doing "Kleihauer betke test" and you give an appropriate dose.
Explained by DJ AweSpear.