Rhesus

Blood types

Not everyone's blood is the same. Blood is classified into a number of different blood groups, the most well known being the ABO group and the Rhesus group. When someone needs a blood transfusion, it is important that they are transfused with blood from the same group as their own. Otherwise a reaction to the ‘foreign’ blood may occur.

The ABO group is identified by a letter of the alphabet, so a person can belong to the A, B, AB or O blood group.

Blood groups also have a plus or minus sign after the letter. Hence someone's group may be ‘O positive’ (written O+) and another ‘AB negative’ (written AB-).

The Rhesus factor

The plus or minus sign after the letter refers to the presence or absence of a substance in blood known as the Rhesus factor, so named because it was first discovered in Rhesus monkeys.

Most people are Rhesus positive (Rh+). But if a Rhesus negative (Rh-) person receives Rh+ blood, their body reacts, making antibodies against the Rhesus factor. This is similar to the way we make antibodies to various viruses such as rubella (German measles) and chickenpox.

The Rhesus factor and pregnancy

The Rhesus factor is most significant during pregnancy. Before, or at, birth it is common for some of the baby's and mother's blood to mix. If the mother is Rhesus negative, and the baby Rhesus positive, this mixing may cause the mother to produce Rhesus antibodies.

In a future pregnancy these antibodies can pass into the baby's circulation. If the new baby is Rh+ these antibodies could destroy its blood, which could, in the worst case scenario, cause death or serious illness. In other cases, however, the baby may suffer only slight anaemia or jaundice.

Nevertheless, whenever a Rhesus negative woman has a baby, its blood group is checked soon after birth. If the baby is Rh+ the mother will be given an injection, known as ‘anti D’, which is designed to help prevent the formation of Rhesus antibodies. Like all vaccines, ‘anti D’ is not 100 per cent effective in all cases, however, it can help protect the health of future pregnancies for many women.