Rhesus Factor

Rhesus Factor

The Rhesus factor is a special antigen or protein on the surface of red blood cells. This is a protective mechanism which helps the body to recognize its own blood. Everyone inherits genes from their parents which determine their blood group, as well as the tendency to either have these antigens or not.

People are either Rh-positive or Rh-negative. In 85% of the population, the D antigens are present which means that the majority of the population have a positive blood group. In the remaining 15%, they are absent, which means these individuals have a negative blood group. Problems arise when a mother has a negative blood group though her baby is positive. Rhesus Disease, otherwise known as haemolytic disease of the newborn, is a complication which can occur when a mother produces antibodies which attack her Rh-positive baby's red blood cells.

Rhesus Disease can be prevented when an Rh-negative mother is given an injection of a special compound known as Anti D, within 72 hours of her having her baby. It is particularly important that she receives this as her future babies can be affected.

What about the Baby?

If an Rh-negative mother and her Rh-negative partner conceive a baby, then there is no problem. The baby develops a negative blood group as well so no antibodies are produced. Likewise, when an Rh-positive mother conceives a baby with a man who is Rh-negative, there are no problems either.

The good news is that Rhesus Disease in developed countries is very rare. It is standard ante-natal procedure for all mothers very early in their pregnancy to have their blood group tested. If a mother has a negative blood group, this is noted in her chart and further blood testing for antibody screening is done at around 28 weeks of gestation.

What are the Different Blood Types?

Blood groups come in either A, B, AB or O
The Rhesus factor is what is added onto the blood group, e.g. A negative, (A-) B positive (B+) and so on.

Genes work in pairs. Fathers with a positive blood group can carry a positive and a negative gene. Because positive is dominant, his blood group will classify him as positive. But any children he fathers will have an equal 50:50 chance of being either positive or negative. But if he has two positive genes, then all of his children will be Rh-positive themselves.

How Does Rhesus Disease Happen?

When a mother who is Rh-negative comes into contact with her baby's Rh-positive blood, she can potentially have an immune reaction. It?s not only during labour when this can happen. Though in a healthy and normal pregnancy, there is no blood to blood contact between a mother and her baby until delivery.

Rh-negative mothers who have had vaginal bleeding during their pregnancy or who have their pregnancy terminated can still be exposed to positive foetal blood. In the same way that when individuals are allergic to particular foods they have an allergic response, this is what happens when a mother?s body reacts to the foreign antigens of her baby's blood.

The mother?s body sends out antibodies, called anti-D antibodies, to attack the baby's Rh-positive cells which have spilled over into her system. If, at a future pregnancy her baby is Rh-positive then these anti-D antibodies cross the placenta and attack the baby's red blood cells.

Treatment of Haemolytic Disease of the Newborn

Babies with this disorder are born anaemic and jaundiced. e.g. they have insufficient iron stores and a high level of bilirubin in their blood. This means they often need a blood transfusion to bring their iron levels up to within a healthy and normal range. This is a carefully planned procedure where small, measured amounts of the baby's blood are replaced with healthy donor blood. This means that the anti-D antibodies are no longer present and the new red blood cells can work as they need to.

What Can Happen?

Babies may also require phototherapy treatment and close monitoring of their iron and bilirubin levels to ensure they are dropping. If haemolytic disease of the newborn is significant, it can potentially cause miscarriage or for the baby to be stillborn. High levels of bilirubin can cross from the blood to the brain and potentially cause brain damage.

What's an Injection of Anti-D?

This is an injection which is given to Rh-negative mothers within 72 hours of giving birth. This prevents the mother from producing anti-D antibodies which could potentially cause problems with her future pregnancies. Even so, a blood test to determine the level of anti-D antibodies is routinely done during subsequent pregnancies for all Rh-negative mothers.