Why is ABO incompatibility not as severe as Rh incompatibility in pregnancy?

The antibodies involved in ABO incompatibility are mainly of IgM variety and they are HUGE in mass and size, so they’re rarely able to traverse the placental barrier (if at all). On the other hand, antibodies involved in Rh incompatibility are mainly of the IgG variety which can easily traverse the placental barrier. Hence the latter is more serious.

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