Rh-isoimmunization

In our last post we discussed what it means to be 'Rh-negative' in response to a patient’s concern that she is. We also mentioned that complications related to being Rh-negative have been greatly reduced over the past 40 years by the prophylactic use of Rh-immunoglobulin during pregnancy (and after delivery) to help prevent ‘isoimmunization’ in Rh-negative women. Isoimmunization, also known as ‘sensitization,’ refers to the development of antibodies to Rh-antigens when women who are Rh-negative are exposed to a source of Rh-positive blood, red blood cells (RBCs) that have the Rh-antigens on their surface and are, therefore, ‘foreign’ to the woman’s immune system. These antibodies do not affect the woman, but may cross the placenta and deleteriously affect the current, or a subsequent, baby.

The most common source of RBC exposure during pregnancy is from the baby. A woman can be exposed to her baby’s RBCs during spontaneous miscarriage (rare) or elective abortion (more common); placental abruption (premature separation of the placenta from the wall of the uterus); fetal-maternal hemorrhage as the result of disruption of the usual ‘barrier’ between the mother’s and the baby’s circulation, occurring with placental abruption, placenta accreta (growth of the placenta into the muscle of the uterus rather than just into the lining), or from other forms of damage or trauma to the placenta; and, most commonly today, as the consequence of exposure to large amounts of fetal blood at the time of a cesarean delivery. Women can also develop Rh-antibodies as the result of transfusion with mismatched blood. The latter is rare today, but is a common cause of maternal isoimmunization to other blood group system incompatibilities that are usually not screened for, particularly when blood is required in emergency situations.

When a person is found to be isoimmunized to Rh (or any other blood group system), the amount of antibody present, or degree of isoimmunization, is usually expressed as an antibody ‘titer.’ A titer is determined by how many times the blood can be diluted and still have the antibody detectable. In this context, the lower the number, the fewer antibodies are present. At the risk of oversimplification, in the case of Rh-isoimmunization, antibody titers less than 16 (or sometimes described as 1 to 16) are rarely associated with severe fetal complications during pregnancy; and, the higher the titer, the greater the likelihood of fetal complications. Baby’s are also at increased risk if the maternal antibody titer rises during the pregnancy or if the mother has previously had a severely affected baby. Usually, if a woman is found (or known) to be Rh-sensitized, she can expect to have monthly antibody titers performed during the course of her pregnancy. If maternal titers are low, and remain low, the baby may be Rh-negative, or may simply be at very low risk and no significant interventions may be required during the pregnancy. If the titers are rising, the baby is more likely to be Rh-positive and, therefore, ‘at risk’ and the pregnancy may require more intensive surveillance, including noninvasive or invasive assessment of the fetal status during the pregnancy.

In my next post, I will discuss the complications the baby may develop as the result of maternal Rh-isoimmunization, and also the options for fetal assessment of risk for these complications…

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