The immune system: Antigens and antibodies.
From the moment of birth, we are introduced into a world of runny noses, dirty hands, bacteria, viruses and other infections. Each of these can impact our health by causing disease, illness and even death. The human immune system enables individuals to resist infections and thrive in what could otherwise be considered a hostile environment.
The immune system works in a remarkable way. Each of an individual’s cells is labeled with small protein tags that help the body to recognize what belongs to “self” and what is considered “foreign.” These protein tags are referred to as antigens. When the body encounters foreign antigens it reacts by developing proteins called antibodies which help the immune system attack and destroy the foreign substance.
Explaining blood types: ABO and Rh type.
An expectant patient is routinely tested for her blood type during pregnancy. Different blood types are determined by the protein tags or antigens that coat a patient’s blood cells. These antigens are divided into two groups. The ABO antigen group represents only part of a patient’s blood type and determines if a patient’s blood type will be “A,” “B,” “AB,” or “O.” Of the second group of antigens, only the “D” antigen is routinely tested. If a patient tests positive for the “D” antigen, they are considered Rh positive. If they test negative for the “D” antigen, they are referred to as Rh negative. When these two test results are combined, they determine a patient’s blood type, such as “A negative,” or “B positive.”
How does being Rh negative affect my pregnancy?
During pregnancy, some fetal blood cells may occasionally escape into the mother’s circulation, exposing her to potentially Rh positive blood cells. If this occurs, an Rh negative mother’s immune system will recognize these cells as foreign and produce antibodies against them called anti-D antibodies. These anti-D antibodies may be produced in sufficient quantities to attack an Rh positive baby’s red blood cells, affecting the baby’s health.
What is RhoGAM?
Rho(D) Immune Globulin, also known as RhoGAM, is a preparation of anti-D antibodies that recognize the “D” antigen as foreign. When this preparation is given to an Rh negative mother it coats Rh positive fetal blood cells in her circulation resulting in their destruction before the mother can produce her own antibodies against them.
Before the administration of RhoGAM, the mother is first screened for the presence of anti-D antibodies with a simple blood test. If she has not yet developed any anti-D antibodies of her own, RhoGAM is administered at 28 weeks and again within three days of delivery if the newborn’s blood type is returned as Rh positive. If the newborn’s blood type is found to be Rh negative, there is no need for the second injection after delivery. If an expectant patient undergoes invasive testing, experiences vaginal bleeding or sustains abdominal trauma, an additional RhoGAM administration is often recommended.
Since the majority of men are Rh positive, testing a father’s blood type has not been considered a cost-effective medical practice. However, if the father of the baby already knows he is Rh negative from previously donating blood, the mother will not need to receive RhoGAM since an Rh negative man cannot father an Rh positive baby.
Alloimmunization: What then?
In spite of the availability of RhoGAM to Rh negative women, 0.1 to 0.2 percent of Rh negative patients still become sensitized to an Rh positive baby and develop anti-D antibodies. These anti-D antibodies pose a risk to future pregnancies since they may attack an Rh positive fetus. This process of becoming sensitized to other human antigens is referred to as alloimmunization. Subsequent pregnancies of alloimmunized mothers are considered high-risk, since these antibodies can cross the placenta causing possible fetal anemia, heart failure, jaundice, neurologic complications or even stillbirth. In these pregnancies, additional testing will be required to ensure the health of the baby and to avoid fetal complications.
For Rh negative mothers, the old addage “an ounce of prevention is worth a pound of cure” is appropriate. It is important for Rh negative mothers to receive RhoGAM administration at the appropriate times during pregnancy to avoid the occurrence of alloimmunization. In addition, if an Rh negative patient experiences any vaginal bleeding or abdominal trauma, it is important that she notify her physician. The need for additional RhoGAM administration can be discussed further with her doctor at that time.