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-).
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.
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.
Last Reviewed: 01 May 2002