The feto-maternal blood incompatibility indicates the transplacental passage of antibodies.
Through lesions in placental vessels or hemorrhages during delivery, small amounts of fetal blood can enter into the maternal circulation. Contamination of the maternal blood can also occur with a miscarriage or a previous transfusion. When the baby is Rhesus-positive (Rh+) and the mother Rh-, the passage of fetal blood into the maternal circulation system can lead to the formation of antibodies against Rh+ antibodies in the mother (agglutinin anti D of type IgM then IgG).
With a subsequent pregnancy the IgG antibodies that are now present in the maternal blood can pass through the placental barrier and thus get into the fetal blood. When the fetus is Rh+, the antibodies bind to the surface of the fetal erythrocytes and destroy them (hemolysis).
This happens in roughly 5% of all pregnancies where the mother is Rh- and the father is Rh+.
The destruction of the red blood corpuscles is responsible for the morbus haemolyticus neonatorum (fetal erythroblastosis) with the following symptoms:
- anemia (due to the hemolysis)
- splenomegaly (location of the macrophages that destroy the erythrocytes)
- hepatomegaly (intensive hematopoesis in order to compensate the hemolysis)
- icterus (transformation of hemoglobin of the destroyed erythrocytes into bilirubin)
The systematic check for Rh antibodies in women in whom such a risk is present permits a prevention of this feto-maternal immunization. When at birth it is established that the newborn is Rh+ , anti-D agglutinin is injected into the Rh- mother, thus neutralizing the fetal erythrocytes that have entered the maternal circulation system.
It is still to be emphasized that feto-maternal immunization reactions can also happen against other surface features of the erythrocytes, e.g., at the ABO system.