Hemolytic disease of the newborn

Hemolytic disease of the fetus and newborn happens when the mother's antibodies destroy her baby's red blood cells. Hemolytic means involving the breakdown of red blood cells. Antibodies are defense proteins the body makes against whatever it recognizes as foreign, and the underlying problem here is a blood group incompatibility: the mother's blood and the baby's blood are not the same type.

In Rh incompatibility, the mother is Rh negative and the baby has inherited the Rh factor from the father. The mother's defense system sees that antigen, a marker on the surface of the baby's red blood cells, as something foreign. The first time, it makes IgM antibodies, which do not cross the placenta. But when it meets that antigen again in a later pregnancy, it rapidly makes IgG antibodies, which do cross the placenta and destroy the fetus's erythrocytes, another name for red blood cells.

When those cells break apart, they leave behind bilirubin, a waste product that has to be cleared. While the baby is in the womb, the placenta carries it away. After birth, that job falls to the baby's liver, which is still immature and is overwhelmed by so much excess bilirubin, like a small sink with more water pouring in than it can drain.