Definitions and terminology

The alloimmune cytopenias are a group of conditions in which the life span of fetal blood cells or their precursors is shortened by the action of antibodies derived from the mother by placental transfer. Three conditions are recognized: antibodies to fetal red cells, platelets or neutrophils, or their precursors, cause alloimmune anaemia, thrombocytopenia or neutropenia, respectively. Various terms are in common usage for these disorders, many of them inappropriate. For example, alloimmune anaemia is sometimes referred to as Rhesus disease, erythroblastosis fetalis or haemolytic disease of the newborn and all three are misnomers. It is incorrect to use ‘Rhesus’ to refer to the Rh system, fetal haemolysis may be caused by antibodies outside the Rh system, anaemia is not always associated with erythroblastosis and, finally, the disorder primarily affects the fetus rather than the newborn. Therefore, throughout this book, alloimmune anaemia (perhaps the best term) will be referred to as haemolytic disease of the fetus and newborn (HDFN). For similar reasons, alloimmune thrombocytopenia and alloimmune neutropenia will be used in preference to other terms, such as neonatal alloimmune thrombocytopenia, fetomaternal alloimmune thrombocytopenia and neonatal alloimmune neutropenia, while, at the same time, acknowledging that these terms are also commonly used.