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What Is Rh Factor and Why Is It Important in Pregnancy?

All of the cells in our body carry small molecules on the outside surface of the cell to help in communication with other cells. These molecules are called antigens. Two of the biggest of these antigens are the “A” antigen and the “B” antigen that determine your blood type. If your blood cells carry the A antigen, your blood type (also called blood group) is “A,” and if they carry the B antigen, your blood type is “B.” If they carry neither the A nor the B antigen, your blood type is “O”. If they carry both, then your blood type is AB. Another antigen important to red blood cells is the Rhesus, or Rh, antigen, so named because it was first discovered in Rhesus monkeys. If your red blood cells carry the Rh antigen, you are deemed to be Rh positive, if not, you are Rh negative. So, if your primary blood type is “O” and you carry the Rh antigen, your blood type is “O positive” (this is actually the most common blood type in the United States). If your blood group is AB and you do not carry the Rh factor, your blood type is AB negative (this is the rarest blood type in the U.S.). The chances of being negative for the Rh factor vary by your ethnic background. For example, about 15% of Caucasians are Rh negative whereas less than 1% of Asians are Rh negative.

Why does this all matter? Well, if a mother is Rh negative and her fetus inherits an Rh gene from its father, then the baby will be Rh positive. Not so infrequently during pregnancy, a little of the baby’s blood can leak into the circulation of the mom’s blood. Most often, the amount of blood is minuscule. But, if the amount of blood is more substantial, the mother’s immune system will look at these cells and realize that they are different from hers, and mount an immune response—a process known as sensitization. Usually when this first happens, there is no affect on that pregnancy. But the immune system has memory so that if more red blood cells leak into the mother’s circulation from another pregnancy and they are Rh positive too, then the mother's immune system sends out an army of antibodies to destroy these cells that are “foreign” to her body. These antibodies towards the Rh factor can cross over the placenta and enter into the baby’s circulation. Since the baby is Rh positive and the antibodies that crossed the placenta are against the Rh antigen, they can destroy the red blood cells in the fetus and cause the fetus to become anemic. The degree of anemia can be mild, which has only minor consequences to the baby (like mild jaundice, or yellowing of the skin), but it can also be severe—to the point that doctors have to give the fetus a red blood cell transfusion while it is still in the womb (this is called an intrauterine transfusion).

Fortunately, there is a way of preventing sensitization. Rh negative mothers can be given a shot of anti-Rh antibodies (also called Anti-D globulin, because this is the most common part of the Rh antigen involved in sensitization) that will prevent the mother from becoming sensitized. It is routinely given at about 28 weeks gestation and at the time of delivery. It is also given at the time of chorionic villus sampling (CVS), amniocentesis or other obstetrical procedures where there might be a risk of leakage of fetal blood into the mother’s circulation. If the anti-D globulin is not given to an Rh negative mother, the chances of becoming sensitized are about 17%, whereas if it is given at the appropriate times, the chances fall to less than 1%. Since the discovery that anti-D globulin prevents sensitization, the incidence of babies being severely affected by this have fallen dramatically.

2 Comments Post a Comment
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by Trippermagee, Apr 04, 2014
I am Rh negative and also have a diagnosis of IITP and I'm  just wondering if the two are related. If so what are the possible side effects.

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by Keith Eddleman, M.D.Blank, Apr 08, 2014
Rh and ITP are not related to each other.  Rh alloimmunization is a problem with maternal antibodies attacking fetal red blood cells.  ITP is a disorder where you make antibodies that attack and destroy your own platelets.  Sometimes (rarely) some of those antibodies can cross the placenta and affect the baby's platelets too.

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