People who are rhesus positive or RhD positive have a protein known as D antigen on the surface of their red blood cells. People who do not have the D antigen are known as RhD negative. Most people are RhD positive but this varies slightly depending on your ethnic origins:
-85 per cent of people of white European origin are RhD positive
-94 per cent of people of African origin are RhD positive
-90 per cent of people of Asian origin are RhD positive
Rhesus status only matters if an RhD-negative mum is carrying an RhD-positive baby (the child inherits this from an RhD-positive dad). If some of your baby's blood gets into your own bloodstream, your immune system may react to the D antigen in the baby's blood as if it were a foreign invader and produce antibodies against it. This is known as sensitising.
Sensitising is not usually harmful in a first pregnancy, but when you become pregnant again, and if your new baby is RhD positive, the antibodies in your system can cross the placenta and attack the blood cells of your baby, causing anaemia, jaundice or in severe cases, heart or liver failure. This condition is called haemolytic disease of the newborn (HDN).
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