Incompatibility of Rh Factor – Erythroblastosis Foetalis

Rh incompatibility becomes medically important specifically in pregnancy, and specifically when the mother is Rh-negative and the father is Rh-positive, because the foetus can then inherit the father's Rh-positive status. During childbirth, damage to blood vessels at the placental interface lets some of the Rh-positive foetal blood cells enter the Rh-negative mother's circulation for the first time. Her immune system recognises the foreign Rh (D) antigen as non-self and becomes sensitised, eventually producing anti-D antibodies — but this sensitisation and antibody production typically only completes after that first child has already been delivered, which is why a woman's first Rh-positive pregnancy usually passes without incident.

The risk appears from the second Rh-positive pregnancy onward. The mother's now-primed immune system produces anti-D antibodies of the IgG class, which (unlike some other antibody classes) are small enough to cross the placenta and enter the foetal circulation directly. There, they attack and destroy the foetus's Rh-positive red blood cells, causing haemolysis that leads to haemolytic jaundice and anaemia in the newborn — the condition called erythroblastosis foetalis, also known as haemolytic disease of the newborn (HDN). …