Erythroblastosis Foetalis (Haemolytic Disease of the Newborn)
The immune system is trained to recognise the body's own cells and to attack anything foreign. Now picture a pregnant woman whose blood is Rh-negative, carrying a baby who is Rh-positive. Her immune system has never met the Rh antigen before, because her own red cells lack it. If even a small amount of the baby's Rh-positive blood leaks into her circulation, her immune system reads it as foreign and starts producing antibodies against the Rh antigen.
That is the seed of the problem — but the first such pregnancy is usually safe. The leak of foetal blood into the mother's circulation typically happens during delivery, when the placenta separates, so by the time her body has built enough antibodies, the baby has already been born. The real danger appears in a later pregnancy.
If the same mother conceives another Rh-positive baby, the antibodies she already carries are small enough to cross the placenta. They enter the foetal circulation and mark the baby's red blood cells for destruction. This immune attack, called haemolysis, makes the foetus anaemic. In response, the foetal bone marrow works overtime and releases immature red blood cells, called erythroblasts, into the bloodstream before they are fully mature — and that premature release is what gives the condition its name, erythroblastosis foetalis.
A common mix-up: this condition is not caused by ABO mismatch between mother and baby. Antibodies in the ABO system are mostly a large type (IgM) that cannot cross the placenta. Rh antibodies are a smaller type (IgG) that can.
The Precise Statement
Erythroblastosis foetalis, also called haemolytic disease of the newborn, occurs when an Rh-negative mother — already sensitised from an earlier exposure — produces IgG antibodies against the Rh antigen of her Rh-positive foetus. These antibodies cross the placenta, bind to foetal red blood cells, and trigger immune-mediated haemolysis. The resulting anaemia drives blood formation outside the marrow (in the liver and spleen) and pushes immature erythroblasts into the foetal circulation.
The sequence, in plain steps: an Rh-negative mother carries an Rh-positive foetus → a small leak of foetal red cells crosses into the mother during a pregnancy or delivery → the mother produces anti-Rh (IgG) antibodies → in a subsequent pregnancy with another Rh-positive foetus, these antibodies cross the placenta → foetal red cells are destroyed → anaemia and erythroblastosis follow.
Why the First Baby Is Usually Unaffected
| Event | What Happens | Consequence |
|---|
| First pregnancy (Rh+ baby) | Foetal red cells leak into the mother mostly at delivery | Mother starts making anti-Rh antibodies only after birth |
| Between pregnancies | Antibody-producing memory cells persist | Mother is now sensitised |
| Second pregnancy (Rh+ baby) | Existing IgG antibodies cross the placenta from early in gestation | Haemolysis can begin before birth |
The first Rh-positive baby is almost never affected. A later Rh-positive baby is at risk only if the mother was already sensitised by an earlier exposure. …