Clearance of Rh-positive red cells by low concentrations of Rh antibody.

Ten previously untransfused Rh(D)-negative subjects were given an intravenous injection of approximately 0.3 ml of Rh(D)-positive red cells and at about the same time, or 24–48 hours previously, were given an intramuscular injection of 1–1000 μg of anti-D. Following the injection of the red cells there was a variable period before the onset of red cell destruction; when the antibody was injected at the same time as the red cells the delay was due partly to the time taken for the antibody to reach the circulation; when the antibody was injected at least 24 hours before the red cells there was still some delay due to the time required for the antibody to be taken up by the red cells; the delay before the onset of a maximum rate of red cell destruction varied from about 0.2 hours following the injection of 1000 μg of antibody to approximately 100 hours following the injection of 1 μg. The maximum rate of red cell destruction was calculated to be approximately proportional to the square root of the amount of antibody on the cells. After the injection of the smallest amount of antibody (1 μg) the amount on the red cells was calculated to be about 0.03 μg/ml corresponding to about ten molecules of antibody per cell. This dose produced clearance with a of the order of 100 hours. The significance of these observations in relation to protection against Rh-immunization is briefly discussed.