The RhD factor & Anti-D
Pregnant women can often belong to different blood groups from their babies. This is perfectly normal and is usually not a problem. However, in about 1 in 10 pregnancies, these blood groups differ in one particularly important way: either the presence or absence of a protein on the surface of red blood cells. This protein was originally called the ‘Rhesus Factor'. It is now referred to as simply the RhD Factor.
If you carry this RhD Factor on your red blood cells, you are known as RhD Positive. If you do not, you are known as RhD Negative.
Sometimes, a small amount of blood can cross over from the baby's circulation in the placenta and enter the mother's blood stream. This can happen at any time in pregnancy, but most importantly during the last 3 months of pregnancy if there is a trauma of some kind, and just before birth. It can also occur at the time of a miscarriage or termination of pregnancy.
If this transfer of blood occurs from a RhD Positive baby to a RhD Negative mother, then the mother's immune system will see the baby's red blood cells as "foreign" and will produce antibodies to get rid of them, but only in the mother's circulation.
The mother's immune system retains the memory of how to make these antibodies, which gives her the ability to make them more quickly and in greater numbers in the future if required.
This generally only becomes a problem during a later pregnancy if the baby is again RhD Positive and there is another transfer of some of the baby's red blood cells across the placenta. The mother's immune system uses its memory to make the same antibodies as before but in quicker and in larger amounts. These can then cross the placenta into the baby's circulation and start to destroy the baby's red blood cells before birth.
Babies who have this problem are said to have Haemolytic Disease of the Newborn, or HDN for short.
Doctors, nurses and midwives are very aware of this problem and can prevent it from happening by giving the mother an injection of immunoglobulin - or ‘Anti-D'. This can rapidly remove any RhD positive red blood cells which pass from baby to mother before they trigger the mother's immune system to produce anti-D. This prevents the mother's immune system developing a ‘memory' for RhD positive red blood cells and later development of HDN in an unborn baby.
Anti-D works by destroying any RhD positive blood from the baby present in the mother's circulation before she can make her own antibodies. This means that the mother does not have the antibodies available to cause HDN in any future pregnancies with an RhD positive baby.
Anti-D is made from a part of the blood called plasma that is collected from donors who have large amounts of anti-D antibodies. It is only from this specific collection of donors that anti-D can be produced: other immunoglobulins do not contain anti-D antibodies. The production of Anti-D immunoglobulin is very strictly controlled to ensure that the chance of a known virus being passed from the donor to the person receiving the Anti-D is very low - it has been estimated to be 1 in 10,000 million doses (i.e. 1 in 10,000,000,000 doses).