Why does coexisting ABO incompatibility between an Rh-negative mother and her Rh-positive fetus reduce the likelihood of Rh isoimmunization?
- A ABO antibodies cross the placenta and destroy fetal red cells before they reach maternal lymphoid tissue in sufficient numbers ✓
- B ABO-incompatible fetuses express weaker D antigen on their red cell surfaces
- C Maternal anti-A and anti-B antibodies bind the fetal cells and block access of anti-D producing B cells
- D ABO incompatibility accelerates clearance of fetal cells by the maternal liver without stimulating memory B cells
Correct answer: A. ABO antibodies cross the placenta and destroy fetal red cells before they reach maternal lymphoid tissue in sufficient numbers
Explanation
Naturally occurring maternal anti-A and anti-B antibodies are predominantly IgM but include an IgG fraction that crosses the placenta. These antibodies rapidly hemolyze fetal red cells that enter the maternal circulation, destroying them before they can be presented to the maternal immune system in sufficient quantity to prime anti-D producing clones. This is why Rh sensitization is less common after ABO-incompatible than ABO-compatible Rh-positive pregnancies. Fetal D antigen density is unchanged, and the protection is immunologic clearance, not antigenic masking.
Reference: Williams Obstetrics, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.