Erythroblastosis Foetalis
if mother is O-


That means:


group O in ABO system: anti-A and anti-B antibodies
group Rh- in Rh system: no D antigen and no ant-D antibodies unless previously senzitized



  baby is any +ve blood group other than O+



ABO system: A, B or both antigens present
Rh system: D antigen present


So, the mother has anti-A and anti-B antibodies and her immune response will be directed towards the fetal RBCs containing A, B and D antigens. But, in theory, the immune system should also produce anti-D antibodies because of antigen presenting cells which will expose D antigens to T lymphocytes.

What is known is that if the mother already has anti-D antibodies, no other are secreted from fetal D antigens. This is how passive immunization vaccines work:


  This is done so that the fetal Rhesus D positive erythrocytes are destroyed before her immune system can discover them. This is passive immunity and the effect of the immunity will wear off after about 4 to 6 weeks (or longer depending on injected dose) as the anti-Rh antibodies gradually decline to zero in the maternal blood [1].


But the mechanism is not clearly understood:


  The most fascinating aspect of the story scientifically is that we still do not have a complete, mechanistic understanding of how this therapy works. The original hypothesis that Rh-immune globulin functions by clearing fetal Rh(D)-positive red blood cells from the maternal circulation, thereby hiding this foreign antigen from the maternal immune system, is inconsistent with the fact that passive immunization with Rh-immune globulin guides the fetal red blood cells directly to antigen presenting cells, the same cells responsible for initiating an immune response to foreign antigens [2].




References:


Wikipedia contributors, "Rh disease," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Rh_disease&amp;oldid=615938189 (accessed July 28, 2014).
Eldad Hod. A Treatment for RH Disease. Available from http://pathology.columbia.edu/rh_disease.html (accessed 28.07.2014)