Describe Rh isoimmunisation and anti-D prophylaxis — its aetiology, clinical features and management.

Definition

Rh isoimmunisation is the development of maternal antibodies against the Rhesus (D) antigen when an Rh-negative mother is exposed to Rh-positive fetal red cells. The resulting anti-D antibodies cross the placenta in a subsequent pregnancy and destroy fetal red cells, causing haemolytic disease of the fetus and newborn (erythroblastosis fetalis).

Sensitising Events

Delivery, abortion or ectopic pregnancy, antepartum haemorrhage, amniocentesis/chorionic villus sampling, external cephalic version, abdominal trauma and any feto-maternal haemorrhage.

Consequences

In the affected fetus — progressive anaemia, hydrops fetalis and intra-uterine death; in the neonate — anaemia and severe jaundice with risk of kernicterus.

Anti-d Immunoglobulin Prophylaxis

• Give anti-D Ig 300 µg IM within 72 hours of delivery to every non-sensitised Rh-negative mother who delivers an Rh-positive baby (cord blood Rh-positive, mother's indirect Coombs test negative).
• Routine antenatal prophylaxis at 28 weeks (± 34 weeks).
• After any sensitising event in pregnancy, and after abortion/ectopic (a smaller dose, e.g. 50–100 µg, suffices in early pregnancy).
• A Kleihauer–Betke test quantifies large feto-maternal bleeds so the dose can be increased.

Management of an Already-sensitised Pregnancy

• Indirect Coombs test (antibody titre) — monitored serially; a critical titre of 1:16 or a rising titre indicates significant risk.
• Middle-cerebral-artery peak systolic velocity (MCA-PSV) Doppler — the non-invasive method to detect fetal anaemia.
• Amniocentesis (spectrophotometry — Liley/Queenan zones) or cordocentesis for direct fetal haematocrit where indicated.
• Intra-uterine (intraperitoneal/intravascular) transfusion for the severely anaemic preterm fetus, and timely delivery with neonatal care (phototherapy, exchange transfusion) when mature.

A Note on WHY Anti-d Works — and WHY Timing Is Everything

A useful concept is understanding the elegant mechanism by which anti-D prevents isoimmunisation, and why it must be given before the mother's immune system has responded. When fetal D-positive red cells enter the circulation of a D-negative mother — at delivery, or at any sensitising event (miscarriage, abortion, amniocentesis, ECV, abdominal trauma, APH) — her immune system recognises them as foreign and mounts an antibody response. Once formed, these IgG anti-D antibodies are permanent, cross the placenta in every subsequent pregnancy, and destroy the red cells of any D-positive fetus — causing haemolytic disease, hydrops and death. Anti-D immunoglobulin works by interference: the injected antibody rapidly coats and clears the fetal cells from the maternal circulation before her own immune system has time to recognise and respond to them — so no immunological memory is formed. This is why timing is everything: anti-D must be given within 72 hours of the sensitising event, and it is useless once the mother has already been sensitised — once she has her own antibodies, nothing can undo it. Understanding that anti-D pre-empts rather than reverses the immune response explains both its dramatic effectiveness and its absolute dependence on being given in time.

Anti-D within 72 hours prevents sensitisation.

• Rh isoimmunisation: a D-negative mother forms anti-D IgG against fetal D-positive red cells → haemolytic disease in later pregnancies (anaemia, hydrops, kernicterus).
• Anti-D immunoglobulin clears fetal cells before the mother's immune system can respond — it prevents and cannot reverse sensitisation.
• Give within 72 hours of any sensitising event and after delivery of a D-positive baby (300 µg / 1500 IU); routine antenatal prophylaxis at 28 weeks.
• Quantify the fetomaternal haemorrhage with the Kleihauer-Betke test to decide whether extra doses are needed.
• Monitor a sensitised pregnancy with indirect Coombs titres and MCA Doppler (peak systolic velocity) for fetal anaemia → intrauterine transfusion.