Clinical overview
Rhesus (Rh) isoimmunisation — more precisely, red-cell alloimmunisation — is the process whereby a pregnant woman who lacks a particular red-cell antigen is exposed to fetal red cells carrying that antigen, mounts an immune response, and produces IgG antibodies that cross the placenta and destroy fetal and neonatal red cells. The clinical consequence is haemolytic disease of the fetus and newborn (HDFN): progressive fetal anaemia, hydrops fetalis, intrauterine death, and severe neonatal jaundice with the risk of kernicterus. The dominant culprit historically is the RhD antigen, hence "Rh isoimmunisation", but the same disease can be caused by anti-c, anti-Kell (anti-K), anti-E and many other antibodies — and Kell sensitisation can be more dangerous than D because it also suppresses erythropoiesis.
This is one of obstetrics' great prevention success stories. Routine anti-D immunoglobulin prophylaxis has reduced RhD sensitisation in developed health systems from a major cause of perinatal loss to a rare event. In South Africa, however, prevention is uneven: anti-D supply, the timing of cover for potentially sensitising events, and access to specialist fetal-medicine monitoring all vary by level of care. The registrar must therefore understand both the immunology (to prevent sensitisation reliably) and the fetal-medicine pathway (to rescue the already-sensitised pregnancy). HDFN sits alongside Antepartum haemorrhage and Hypertension in pregnancy as a complicated-obstetrics problem where booking bloods, vigilance for sensitising events, and timely referral change outcomes. Anti-D prophylaxis and red-cell antibody management are anchored in the SA National Integrated Maternal and Perinatal Care Guideline (NDoH, 2024), with international detail from NICE TA156, BSH red-cell-antibody guidance and ISUOG Doppler standards.
Core knowledge
The Rh blood group system
The Rh system is coded by two adjacent genes on chromosome 1: RHD (encoding the D antigen) and RHCE (encoding C/c and E/e antigens). "Rh-positive" means the D antigen is present; "Rh-negative" means it is absent — most commonly because the entire RHD gene is deleted. RhD negativity is a recessive phenotype. Antigen prevalence is strongly population-dependent: roughly 15% of people of European ancestry are RhD-negative, but the proportion is considerably lower in most African and Asian populations. In the South African setting the mix of ancestries means RhD-negative women are a clinically important minority in every booking clinic, so every booking visit must include blood group and antibody screening (standard SA antenatal practice; SA Maternity Guideline (NDoH, 2024)).
How sensitisation happens
A woman becomes alloimmunised when fetal red cells carrying an antigen she lacks enter her circulation in sufficient quantity and provoke antibody formation. The largest fetomaternal haemorrhage (FMH) typically occurs at delivery, but sensitising events occur throughout pregnancy: miscarriage, ectopic pregnancy, Termination of pregnancy, antepartum haemorrhage, abdominal trauma, external cephalic version, invasive procedures (amniocentesis, chorionic villus sampling), and intrauterine death. Even a small volume of fetal blood — classically of the order of well under a millilitre — can sensitise.
The first exposure usually generates a primary IgM response that does not cross the placenta, so a first sensitising pregnancy is often spared. The danger is the anamnestic (secondary) response in a subsequent pregnancy: rapid, high-titre IgG production. IgG crosses the placenta actively, coats antigen-positive fetal red cells, and triggers their destruction in the fetal reticuloendothelial system (predominantly the spleen).
From antibody to hydrops
Fetal haemolysis drives compensatory extramedullary haematopoiesis in the liver and spleen, causing hepatosplenomegaly. As anaemia worsens, hepatic architecture is disrupted, portal hypertension and hypoalbuminaemia develop, and high-output cardiac failure ensues. The end stage is hydrops fetalis — abnormal fluid in two or more compartments (ascites, pleural and pericardial effusions, skin oedema, polyhydramnios, placental oedema) — which carries a high risk of intrauterine death.
After birth the placental "clearance" of bilirubin is lost, so the neonate develops rapid, severe unconjugated hyperbilirubinaemia. Unconjugated bilirubin crosses the immature blood–brain barrier and deposits in the basal ganglia, producing acute bilirubin encephalopathy and, if untreated, kernicterus — choreoathetoid cerebral palsy, sensorineural hearing loss and gaze palsy.