The deal with Rh alloimunization is that the first exposure to Rh factor (duffy Ag) creates IgM. If this primary exposure is in pregnancy then IgG won't be in sufficient amounts to cause hydrops in this initial pregnancy because it takes a few months for seroconversion from IgM to IgG but in a second pregnancy with Rh- fetus hydrops will occur b/c IgG will be around and it can cross the placenta. Thats for RH factor!!!
As for ABO blood type- this is a phenomena- ABO antibodies exist only in IgM form and never progress to IgG. Weird but true. Hence the ability to give birth to a fetus with a totally different blood type. The ABO antibodies do form with no previous exposure. weird aint it.
Isn't nature fascinating!