In one line
Red-cell alloimmunisation is a prevention disease first and a fetal-medicine disease second: stop anti-D sensitisation with correctly dosed, correctly timed immunoglobulin, and once a clinically significant antibody exists, drive surveillance off MCA peak systolic velocity (>1.5 MoM) rather than amniocentesis — escalating to intrauterine transfusion in a fetal-medicine centre.
This chapter assumes the Intermediate groundwork — Rh antigen biology, the mechanics of sensitisation, ABO/Rh inheritance and routine prophylaxis — from Rh iso-immunisation basics; it covers which antibody behaves how and why, the tests that mislead, the named transfusion technique, and the judgement calls (including how to ration anti-D in a shortage and genotype a fetus in an African population).
Why this matters in South Africa
The epidemiology here is not the European epidemiology. The D-negative phenotype runs at roughly 3–7% in Black African populations (versus ~15% in Caucasians), so the number of at-risk pregnancies is lower — but two SA realities make the disease disproportionately dangerous. First, anti-D immunoglobulin is chronically short and expensive in the public sector, so the prophylaxis programme that all but eliminated Rh disease in high-income countries is delivered imperfectly here; established severe disease still presents. Second, the molecular basis of D-negativity differs: in Africans the D-negative phenotype is usually caused not by deletion of the RHD gene (the Caucasian mechanism) but by an intact-but-silenced RHD pseudogene (RHDψ) carrying a 37-bp insert and stop codons. This has a direct clinical consequence developed below — non-invasive fetal RHD genotyping that screens only one exon will misclassify these women, so the assay and its interpretation must be African-aware. The combination — a population with non-deletional D-negativity, an unreliable anti-D supply, and send-away fetal genotyping — is exactly why this is a judgement topic in SA, not a protocol topic.
Pathophysiology — why the antibody, not the titre, decides
Assume the two-hit story from Intermediate (a sensitising bleed primes the maternal immune system; on re-exposure, IgG anti-D crosses the placenta and haemolyses antigen-positive fetal cells). The antibodies classify by mechanism, because each subtype produces a different disease and a different management trigger.
- Anti-D — the classic IgG-mediated extravascular haemolysis. Maternal IgG coats D-positive fetal red cells; the fetal reticuloendothelial system (splenic macrophages) destroys them. Fetal anaemia drives compensatory extramedullary haemopoiesis (hepatosplenomegaly), then high-output cardiac failure and, at the extreme, hydrops fetalis (ascites, effusions, skin oedema, placentomegaly) once the haematocrit falls below roughly a third of normal. Here titre tracks severity reasonably well and the disease worsens with each pregnancy (the anamnestic response), so a prior affected pregnancy is one of the strongest predictors.
- Anti-c and anti-E (Rh system, non-D). Anti-c can cause disease as severe as anti-D and is now a leading cause of severe non-D haemolytic disease; anti-E alone is usually mild but the combination anti-c + anti-E is synergistic and more severe than either alone. These matter because RhD prophylaxis does nothing to prevent them — a woman can be diligently anti-D-protected and still alloimmunise to c or E.
- Anti-Kell (anti-K) — a different disease. Kell glycoprotein is expressed on erythroid progenitor cells (BFU-E/CFU-E), not just mature red cells. Anti-K therefore suppresses erythropoiesis at the progenitor level in addition to haemolysing — it is a marrow-failure picture as much as a haemolytic one. The mechanistic consequences: (1) anaemia is more severe than the maternal titre, the antibody history, or ΔOD450 predicts, because there is little haemolysis to generate bilirubin or reticulocytosis to read; (2) because there is less haemolysis there is less amniotic bilirubin, so the old ΔOD450 method systematically underestimates Kell disease; and (3) fetal anaemia can develop early and fast, and rare case reports describe profound anaemia that even weekly MCA-PSV missed. So in anti-Kell, refer and survey on antibody presence and paternal/fetal Kell status, not on titre.
- The non-haemolytic mimics that present as "anaemia + antibody". A positive antibody screen plus fetal hydrops is not always alloimmune. Non-immune hydrops (parvovirus B19 aplasia, alpha-thalassaemia major / Bart's hydrops, fetal arrhythmia, structural cardiac disease, twin–twin transfusion) produces the same MCA-PSV picture and the same ultrasound, but the antibody is a red herring — see the differential below.
The unifying point: the antibody's specificity, not its concentration, sets the rules. A "low titre" is reassuring in anti-D and meaningless in anti-Kell.
Assessment
Assume the booking-bloods routine from Intermediate. Two questions sharpen at this level: is this antibody capable of causing fetal anaemia, and how severe is it likely to be this pregnancy?
- Screen every pregnancy, not just RhD-negative women. The booking indirect antiglobulin (Coombs) test must look for all clinically significant antibodies — anti-D, anti-c, anti-E, anti-C, anti-e and anti-Kell — because non-D antibodies now cause a large share of severe haemolytic disease.
- Know which antibodies matter. Anti-D, anti-c and anti-Kell cause severe disease; anti-E and most others are usually milder. Anti-Kell behaves differently: it suppresses erythropoiesis as well as haemolysing, so anaemia is more severe than the titre or the ΔOD450 predicts, and titre correlates poorly with severity.
- Titre drives the first decision. A critical titre of 1:16 or higher (or any prior affected pregnancy) mandates fetal-medicine referral. Below that, repeat titres 2–4 weekly in the same laboratory — inter-laboratory drift is a real source of error.
- Determine the fetal antigen. If the antibody is present, establish whether the fetus even carries the target antigen. Cell-free fetal DNA (cffDNA) genotyping from maternal plasma is non-invasive and highly accurate; an antigen-negative fetus needs no surveillance at all. In SA this is send-away, costly and not reimbursed by medical aids — so paternal genotyping (with the caveat of non-paternity/new partner) or specialist referral is the pragmatic fallback.
- Surveillance once at risk = MCA-PSV. From 16 weeks, serial middle cerebral artery peak systolic velocity, plotted in multiples of the median (MoM) for gestation. A value >1.5 MoM predicts moderate-to-severe fetal anaemia and triggers fetal blood sampling ± transfusion. Look also for early hydrops (ascites, skin oedema, effusions) — a late, ominous sign.
The advanced judgement: prior history beats every number
The single most useful prognostic variable is the outcome of the last affected pregnancy, and it reshapes the whole surveillance plan. Alloimmune disease tends to present 2–3 weeks earlier and more severely with each successive pregnancy (the anamnestic response strengthens). So a woman whose previous fetus needed transfusion at 28 weeks should start MCA-PSV surveillance well before that gestation in the index pregnancy — typically from 16–18 weeks — rather than waiting for a critical titre, because in a sensitised multip the titre is no longer a useful trigger. As a rule: titre triages the first sensitised pregnancy; history triages every one after it.
Why cffDNA genotyping needs an African-aware assay
For an unsensitised D-negative woman, fetal RHD genotyping can spare anti-D entirely if the fetus is D-negative; for a sensitised woman it can abolish surveillance if the fetus lacks the target antigen. But the SA molecular reality from the opening section bites here. Because most D-negative Africans carry the silent RHD pseudogene rather than a deleted gene, an assay that detects only one RHD exon will read "RHD present" and call the fetus D-positive when it is in fact D-negative — a false positive that wastes anti-D and prompts needless surveillance. Robust non-invasive RHD genotyping in an African population therefore amplifies multiple exons (commonly exons 5, 7 and 10) so that the pseudogene (exon-7-negative) is distinguished from a truly D-positive fetus. Do not order or interpret cffDNA RHD typing in this population without knowing the assay is multi-exon and pseudogene-aware — a single-exon result is uninterpretable here.
The protective effect you can use to read the history
When counselling on the likelihood of sensitisation, note that ABO incompatibility between mother and fetus is partially protective against Rh sensitisation: incompatible fetal cells leaking into a group-O mother are rapidly cleared by maternal anti-A/anti-B before the D antigen can be processed. Quantitatively, the spontaneous sensitisation risk of a D-positive ABO-compatible fetus (no prophylaxis) is roughly 16%, falling to about 1.5–2% when the pair is ABO-incompatible. This explains the otherwise-puzzling D-negative multip who never sensitised, and it is why the residual sensitisation risk in the era of anti-D is not zero — most third-trimester sensitisation is from silent feto-maternal haemorrhage in the weeks before a missed or under-dosed prophylaxis.