Why does haemolytic disease of the newborn not occur in rhesus positive mothers with a negative baby?

Haemolytic disease of the newborn (HDN) does not occur in rhesus positive mothers with a rhesus negative baby because the mother's immune system does not produce antibodies against the baby's rhesus factor.

This is because the rhesus factor is an antigen that is present on the surface of red blood cells.

In rhesus positive individuals, the rhesus antigen is present, while in rhesus negative individuals, it is absent.

In HDN, the mother's immune system, which produces antibodies against foreign antigens, recognizes the rhesus antigen on the baby's red blood cells as foreign and attacks it, leading to the destruction of the baby's red blood cells. This can cause anemia, jaundice, and other complications in the baby.

In the case of a rhesus positive mother, her immune system does not produce antibodies against the rhesus antigen because she is also rhesus positive, i.e. her red blood cells also carry the rhesus antigen. Therefore, her immune system does not recognize the baby's rhesus antigen as foreign, and does not attack it, preventing the development of HDN.