Relationship between maternal anti-D levels, fetal phenotype and haemolytic disease of the newborn.

It has been demonstrated that a lower level of maternal anti-D is required to produce the same symptoms of haemolytic disease of the newborn in R2r infants than in R1r infants. This difference could be explained by postulating that the higher site density of R2r cells is associated with an increased association constant and thus will bind sufficiently more anti-D. The level at which amniocentesis should be considered has been calculated to be 1.44 microgram anti-D/cm3 of maternal serum for R1r infants and 0.73 for R2r infants.