Blood transfusion for haemolytic disease as a cause of leukocytosis in the fetus.

We evaluated the effect of fetal intravascular transfusion (IVT) of leukocyte-poor red blood cells to correct fetal anaemia due to haemolytic disease on the fetal leukocyte count in 153 patients. Initial, mid-transfusion, and closing haematological studies were obtained when possible. The effect on leukocyte subsets was evaluated by the manual differential count. Fetal leukocyte count increased an average of 18.0 per cent during all IVTs (P < 0.01), despite the dilutional effect of the transfusion. The degree of leukocytosis increased with subsequent transfusions. The smallest (10.1 per cent) change occurred during the initial transfusion. A 41.8 per cent increase was noted during the sixth procedure. There was no relation between leukocytosis and gestational age, volume of transfusion, changes in umbilical vein pressure, or hydrops. Forty-two per cent of the increase was due to expansion of the neutrophil pool and 22 per cent was due to expansion of the monocyte pool. The neutrophil count increased 29 per cent (P < 0.01) and monocytes increased 64.7 per cent (P < 0.001). There was no change in eosinophil and lymphocyte counts. Both an immune aetiology and an effect of inflammatory agents may contribute to this leukocytosis.