Haemolytic disease of the newborn, also known as haemolytic disease of the foetus and newborn, HDN, HDFN, or erythroblastosis fetalis, is an alloimmune condition that develops in a foetus, when the IgG molecules produced by the mother pass through the placenta. During pregnancy, IgG antibodies are actively transported across the placenta. If the foetus has the RBC antigen corresponding to the mother’s antibody, the antigen-antibody interaction can result in the destruction of the foetal RBCs.
Among these antibodies are some which attack the red blood cells in the foetal circulation; the red cells are broken down and the foetus can develop reticulocytosis and anaemia. This foetal disease ranges from mild to very severe, and foetal death from heart failure (hydrops fetalis) can occur. When the disease is moderate or severe, many erythroblasts are present in the foetal blood and so these forms of the disease can be called erythroblastosis fetalis.