Rh Isoimmunization and Other Blood Group Incompatibilities

Rh Isoimmunization Basics - Red Cell Rumble

• Definition: Alloimmunization in RhD-negative mother against RhD-positive fetal RBCs, forming anti-D IgG.
• Pathogenesis:
  • Initial exposure: Fetal RhD+ RBCs enter maternal circulation (e.g., delivery, trauma, procedures).
  • Sensitization: Mother produces anti-D IgG antibodies.
  • Subsequent RhD+ pregnancy: Maternal IgG crosses placenta, attacks fetal RBCs → hemolysis, HDFN.

⭐ The D antigen is the most immunogenic Rh antigen; while ABO incompatibility is the most common cause of Hemolytic Disease of the Fetus and Newborn (HDFN), RhD isoimmunization (if unsensitized) can lead to its most severe form.

• 📌 Mnemonic: "Rhesus Hates Duncan" (RhD - D antigen is key).

Antenatal Screening & Diagnosis - Womb Watch

• Booking Visit (All):
  • ABO/RhD typing & Indirect Coombs Test (ICT) for antibody screen.
• RhD-Negative, Unsensitized (ICT Negative):
  • Paternal RhD status.
  • Repeat ICT at 28 weeks, then delivery.
  • Prophylactic Anti-D Ig (e.g., 300 µg) at 28 weeks & post-sensitizing events.
• RhD-Negative, Sensitized (ICT Positive):
  • Identify antibody; serial titers (q2-4 wks). Critical titer: ≥1:16-1:32.
  • Fetal surveillance:
    • Ultrasound: For hydrops fetalis (edema, ascites, effusions).
    • Doppler: Middle Cerebral Artery Peak Systolic Velocity (MCA-PSV).

      ⭐ Middle Cerebral Artery Peak Systolic Velocity (MCA-PSV) >1.5 Multiples of Median (MoM) is the primary non-invasive tool to detect fetal anemia and guide interventions.

    • If MCA-PSV ↑ or hydrops: Consider Amniocentesis ($\Delta OD_{450}$) or Cordocentesis (fetal Hb, DCT).

Antenatal Management & IUT - Fetal Rescue Mission

• Goal: Prevent/treat fetal anemia & hydrops fetalis.
• Indication for Intervention: Severe fetal anemia (e.g., MCA-PSV > 1.5 MoM).
• Intrauterine Transfusion (IUT):
  • Procedure: Ultrasound-guided, typically into umbilical vein.
  • Blood: O RhD-negative, CMV (-), irradiated, packed RBCs (<5-7 days old).
  • Target: Fetal hematocrit 40-50%.
  • Frequency: q1-4 weeks until maturity or delivery.
• Delivery Planning:
  • Timing: 37-38 weeks if stable post-IUTs.
  • Earlier if severe anemia refractory to IUT or complications.
  • Antenatal corticosteroids for lung maturity if delivery <34-37 weeks.

⭐ For Intrauterine Transfusion (IUT), Group O RhD-negative, CMV-negative, irradiated, packed red blood cells (ideally <5-7 days old) are used to achieve a target fetal hematocrit of 40-50%.

Prevention & Postnatal Care - Shield & Heal

• RhD Immunoglobulin (RhIG) for RhD-Negative Unsensitized Mothers:
  • Antenatal:
    • Routine: 300 µg IM at 28 weeks.
    • Post-event (FMH risk, e.g., APH): 300 µg IM.
  • Postpartum (RhD+ baby):
    • Standard: 300 µg IM within 72 hours.
    • Assess FMH (Kleihauer-Betke); may need ↑ dose.

⭐ > A standard 300 µg dose of RhD immunoglobulin (RhIG) neutralizes up to 15 mL of fetal RhD-positive red blood cells (or 30 mL of fetal whole blood) and should be given within 72 hours postpartum to an unsensitized RhD-negative mother delivering an RhD-positive infant.

• Postnatal Care (Affected Neonate):
  • Monitoring: Jaundice, anemia.
  • Labs: Cord blood (Rh, DAT, Hb, Bili).
  • Management:
    • Phototherapy.
    • Exchange Transfusion (severe).
    • IVIG.

High‑Yield Points - ⚡ Biggest Takeaways

• RhD isoimmunization: RhD-negative mother and RhD-positive fetus.
• Sensitization (e.g., delivery) leads to maternal IgG anti-D, causing fetal hemolysis in subsequent pregnancies.
• Results in fetal anemia, hydrops fetalis, erythroblastosis fetalis, kernicterus.
• Prevention: Anti-D immunoglobulin (RhoGAM) at 28 weeks and postpartum.
• Kleihauer-Betke test (KBT) quantifies FMH to adjust Anti-D dose.
• ABO incompatibility: most common, usually milder than Rh.
• ICT detects maternal sensitization; DCT on neonate confirms antibody-coated RBCs.