An Rh-negative mother is carrying her second Rh-positive baby. What attacks the fetal blood in this pregnancy?

1. The Rh antigen is a red blood cell surface antigen carried separately from the ABO blood type. 2. During the birth of a first Rh-positive child, fetal blood enters the maternal bloodstream and the Rh-negative mother makes a primary anti-Rh antibody response. 3. If she carries a second Rh-positive child, a secondary response follows, and secondary responses in blood are dominated by IgG. 4. IgG is also the one class that crosses the placenta. 5. Those IgG antibodies against Rh-positive blood cross the placenta and attack the fetal blood, causing anemia, the disease erythroblastosis fetalis. 6. The fetus mounts no part of this attack, so both options built on fetal antibodies fail. 7. Treatment gives the mother antibodies specific for Rh factor during the first and subsequent births, destroying fetal blood cells before she can respond to them. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 21 "The Lymphatic and Immune System", section 21.7 Transplantation and Cancer Immunology_