Mild course of fetal RhD haemolytic disease due to maternal alloimmunisation to paternal HLA class I and II antigens.

The patient is a pregnant women of African origin with a prior history of spontaneous abortion and newborn dystrophy. The investigation showed an anti-Rh D antibody (IgG isotypes 1 and 3) with an indirect antiglobulin tube test (IAT) titre of 1:512. The monocyte monolayer assay (MMA) proved clearly the interaction of Fc receptors with the maternal anti-D, and so a clinical significance was expected. In spite of this, no signs of severe haemolysis in the Rh-D-positive and direct antiglobulin test-positive fetus could be observed. Furthermore, two HLA class I and II alloantibodies (anti-A10, anti-DR13) directed against paternal and fetal antigens were detected in the serum of the gravida. Both antibodies showed an inhibitory effect on the in vitro phagocytosis capacity of mononuclear cells expressing at least one of the corresponding HLA antigens (immunophagocytosis inhibition (IPI) test). Thus, the mild course of haemolytic disease may be explained by an effective inhibition of the fetal mononuclear phagocyte system by maternal HLA class I and/or class II antibodies resulting in a diminished destruction of anti-D-coated fetal red blood cells.