Epidemiology

Causes and incidence of alloimmunization

Anti-D is still the commonest cause of moderate and severe haemolytic disease of the fetus and newborn (HDFN) in the developed world. Alloimmunization after pregnancy with a D-positive baby is not as frequent as might be expected from the predicted incidence of D incompatibility between father and mother (60% of Dnegative pregnant women carry a D-positive fetus). There are a number of reasons for this. First, the volume of fetal cells crossing the placenta may be too small to initiate a response. Second, ABO incompatibility between mother and fetus offers some protection. Third, the Rh phenotype of the fetus affects antigenicity. Fourth, some women appear to be better responders to a small antigenic stimulus than others. About 17% of at-risk unprotected women are immunized by an ABO compatible pregnancy. Only 8% develop anti-D in the 6 months following a first pregnancy, but a further 9% develop anti-D early in a subsequent pregnancy, indicating that they had been ‘sensitized’ by the earlier pregnancy.

Ethnic groups

About 15–17% of the UK white population is D negative, and a similar approximation can be made for the Caucasian populations of North America, Europe, Australia and New Zealand. The incidence of D negatives is about 10% in Arabs, about 5% in ethnic blacks, 4–10% in Asian Indians and less than 1% in ethnic Chinese. HDFN associated with anti-D is therefore largely a disease affecting Caucasians, and Caucasians thus form a target for cost-effective preventive prophylaxis programmes.