[Hemolysis can conceal another hemolysis].

We report a delayed haemolytic reaction during the course of a severe HELLP syndrome, which required red blood cell transfusions. Haemolysis was mainly ascribed to the erythrocyte alloantibody anti-Fy a. The fact that this antibody was undetectable before transfusions (despite cross-matching and anti-erythrocyte antibody screening test) emphasizes the limits of these tests for low antibody concentrations.