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Rh factor incompatibility
Ordinarily, the antibodies in a mother's bloodstream to foreign type blood are of a kind that is large and does not cross the placental barrier into the bloodstream of the fetus. Aside from the A and B antigens possibly present on the baby's red cells, there is another class of protein called the Rh antigen. If a mother has Rh negative type blood (does not herself possess the Rh antigen on her blood cells), and carries a fetus whose blood type is Rh positive, cells from the baby can migrate across the placental barrier and stimulate the production of a maternal antibody that can cross back into the baby's system and trigger the destruction of the baby's red blood cells. This is termed a hemolytic anemia.
The resultant flood of waste product from the breakdown of these red cells overwhelms the baby's ability to cleanse the bloodstream of this waste product (bilirubin).
If the level of bilirubin is thought to be too high for safety or rising fast enough to cause concerns that the amount of bilirubin in the bloodstream might reach a harmful level, treatment will be instituted - either phototherapy or exchange transfusion.
Usually, the abnormal antibody posed little threat to the first Rh-positive baby born to an Rh-negative mother. It was subsequent Rh-positive babies who were at risk. Happily, this disease is now very rare, thanks to the invention of an immunization given to Rh-negative mothers after delivery of an Rh-positive baby. This shot (RhoGam®) blocks the formation of high levels of the abnormal anti-Rh antibody and protects subsequent pregnancies with a high degree of efficiency.
A similar maternal antibody induced hemolytic anemia (anemia caused by abnormal red blood cell destruction) can also result when mother's blood type is Type O and the baby's is Type A or B; in general, these anemias are not as severe, and rarely cause jaundice severe enough to require exchange transfusion.