Blood groups - Rhesus factor

The Rhesus factor is a protein that is either present or absent on the surface of a person’s red blood cells. Hemolytic disease of the newborn (HDN) can occur if there is Rhesus incompatibility between a mother and her unborn child. The incompatibility can happen if an Rh-negative mother and an Rh-positive father conceive an Rh-positive baby. A woman who is Rh-positive will not be affected by having a baby with an Rh-negative man. HDN is also known as ‘Rhesus disease’, Rh (D) disease or RhD HDN.

In severe cases of HDN, the foetus may die without medical treatment. HDN is now uncommon in developed countries, due to routine ‘anti-D’ vaccination given to the Rh-negative mother after the birth of her first child.

Blood groups explained
The genes you inherit from your mother and father determine your blood group.
Blood is described by the type (A, B, AB or O) and the Rhesus (Rh) factor. The Rhesus factor is a protein that is either present or absent on the surface of your red blood cells. In medical shorthand, this is indicated by a plus sign (+) or a minus sign (-). For example, the blood type O+ means that the blood is type O and each blood cell has the Rhesus factor.

A blood group is identified by a collection of inherited chemical substances (antigens) located on the surface of each red blood cell. The antigens, including the Rhesus factor, help the body to distinguish its own blood from the blood of another person.

Genetics and blood groups
About 85 per cent of people are Rh-positive. The remaining 15 per cent are Rh-negative.

The Rh-negative gene is recessive, while the Rh-positive gene is dominant. This means that there is a greater than or equal to 50 per cent chance that an Rh-negative mother will conceive an Rh-positive baby if the father is Rh-positive.

Genetic testing of the father can help to predict the chance of conceiving another Rh-positive baby. Genes come in pairs. If the father has one Rh-positive gene and one Rh-negative gene, each child has a fifty–fifty chance of being Rh-positive. However, if the father has two Rh-positive genes, all children conceived by the couple will be Rh-positive.

Immune system response
A person who is Rh-negative (whose blood cells do not have the Rhesus factor), may have an immune reaction if Rh-positive blood cells enter their bloodstream. This can happen during a miscarriage, an abortion or childbirth. This is because blood from an Rh-positive baby may enter the bloodstream of an Rh-negative mother.

The mother’s body considers the Rh-positive cells a threat and mounts an immune system response. Her immune system makes antibodies (called anti-D antibodies) against the Rh-positive blood cells. If the mother conceives another Rh-positive baby, her anti-D antibodies will attack her unborn baby’s red blood cells. This complication of pregnancy is called haemolytic disease of the newborn (HDN) or ‘Rhesus disease’.

A pregnancy affected by Rhesus disease should be closely monitored. In some cases, medical treatment may include one or more blood transfusions for the unborn baby.

HDN affects the second baby
HDN rarely affects the first pregnancy. However, HDN can affect any later Rh-positive foetuses.

In severe cases, the unborn baby’s blood cannot carry oxygen and the baby dies of heart failure. This explains why foetuses affected by HDN may be miscarried or stillborn.

Babies can survive
If the baby survives, it may be born with anaemia and jaundice. Jaundice causes a yellowish tinge to the skin and eyes, caused by an excess of the chemical called bilirubin. The baby’s liver makes bilirubin as it breaks down the damaged or destroyed red blood cells. High levels of bilirubin can be toxic.

However, some Rh-positive babies born to Rh-negative mothers are either healthy or have mild anaemia that is easily treated.

Treatment
The preferred medical treatment for a baby born with HDN is blood transfusion. The baby’s damaged blood is regularly replaced in small amounts with compatible donor blood. The blood exchange continues until the baby’s blood has been completely replaced with healthy donor blood. The transfusion also removes maternal anti-D antibodies, which prevents damage to the baby’s new red blood cells.

Prevention
A vaccine against HDN has been available for about 40 years. The vaccine helps to prevent the mother’s immune system from making anti-D antibodies and offers protection for future pregnancies against Rhesus disease. Studies show that about 99 per cent of Rh-negative mothers do not make anti-D antibodies after receiving the vaccination.

Since vaccination was first introduced, the death rate from HDN in developed countries has dropped significantly. The vaccine, which contains anti-D immunoglobulin, is given as an injection to the Rh-negative mother during pregnancy and after she gives birth to an Rh-positive baby.

Rh-negative women should also be vaccinated if there has been any possibility of foetal blood entering their bloodstream. For example after:

• Miscarriage
• Abortion
• Amniocentesis
• Abdominal trauma during pregnancy
• Manipulation of a breech presentation during delivery.

• Your doctor
• Obstetrician
• Royal Women’s Hospital Tel. (03) 8345 2000

• The Rhesus factor is a protein that is either present or absent on the surface of a person’s red blood cells.
• HDN or ‘Rhesus disease’ can occur if there is Rhesus incompatibility between a mother and her unborn child.
• Vaccination of the Rh-negative mother during pregnancy or shortly after giving birth to her Rh-positive baby helps to protect future pregnancies against HDN.

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