A term neonate presents on day 4 with jaundice. Total bilirubin is 18 mg/dL (unconjugated). Mother's blood group is O positive, baby's blood group is A positive. Direct Coombs test is weakly positive. Which mechanism BEST explains the hemolysis in this case?
- A IgG anti-A antibodies crossing placenta and coating fetal erythrocytes ✓
- B IgM anti-A antibodies activating complement on fetal erythrocytes
- C Anti-D IgG antibodies crossing placenta from sensitized mother
- D Deficiency of UDP-glucuronosyltransferase in the neonate
Correct answer: A. IgG anti-A antibodies crossing placenta and coating fetal erythrocytes
Explanation
ABO incompatibility in this context occurs because group O mothers naturally carry IgG subclass anti-A and anti-B antibodies that cross the placenta, unlike IgM antibodies which cannot. These IgG antibodies coat group A or B fetal erythrocytes, leading to extravascular hemolysis by macrophages in the reticuloendothelial system. The reaction is typically milder than Rh disease because A and B antigens are also expressed on non-erythroid tissues, reducing antibody availability.
Reference: Ghai Essential Pediatrics, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.