Rh Factor (Part 1): A Primer
All of our blood cells carry specific inherited markers or antigens. There are many groups or classes of antigens, but the two major types are the “ABO” blood type antigens and the Rh or “CDE” blood group system. These antigens are inherited and are the result of contributions from both parents. As embryos develop, the immune system comes to recognize these antigens as “self” or ok. However, if different antigens are introduced, the body will become sensitized to these foreign antigens, and then with any subsequent exposure mount a very aggressive attack against any cells that bear these foreign marks (antigens). Thus the importance of typing your blood prior to receiving a transfusion -the effort is made to make sure the antigens in the donated blood match those in your blood. The potential for mismatching also occurs in pregnancy as the fetus’ blood type is the product of both parents, it may not match that of the mother’s. The CDE or Rh blood group system is most likely to cause an issue in this circumstance and is referred to alloimmunization. This occurs when a mother who is Rh negative is exposed to Rh(+) blood. Typically, the first time this occurs, the fetus is unaffected. However, any subsequent pregnancy will result in hemolytic disease of the fetus and newborn (HDFN).
The Rh antigen group is derived from 3 genetic loci (genes), each with two different major alleles (types). Anti-sera were originally used to identify the antigens produced by these genes and were labeled C, c, D, E, e ( Fisher–Race nomenclature). No anti-sera was found for type “d” so for the D allele, either it is present or absent. For this reason, the Rh group can produce eight different antigen profiles, with CDe and cde being the most common in Caucasians. The vast majority of cases of HDFN are a result of incompatibility with the D antigen. The D antigen became known as Rh factor as the antibody against the D antigen was first produced by immunizing Rhesus monkeys to red blood cells from a rabbit. Rh positive then indicates the presence of the D antigen on red blood cells and Rh negative indicates its absence. In addition to the five major antigens of the Rh blood group, over 30 other variants have been identified. One especially important variant is Cw and Du antigen that are also known as weak D. Some patients with weak D are capable of developing anti-D antibodies but this rarely develops into HDFN.
A Rh negative female who mates with a Rh positive male is at risk of developing HDFN in a future pregnancy. The risk of an incompatibility is related to race and if the male is homozygous or heterozygous for Rh. If the baby is Rh (+), alloimmunization can occur if a sufficient number of red cells gain access to the circulation of a Rh negative mother. The risk of a fetal-maternal transfusion is highest at birth, but it can occur anytime during pregnancy either silently or in conjunction with a documented bleeding event. In addition, early pregnancy loss and ectopic pregnancies will put patients at risk. The volume necessary to cause this reaction varies, and in 3% of women, exposure to a transplacental bleed of <0.1ml can result in alloimmunization.
Prior to 1970, the incidence of RhD alloimmunization and resultant HDFN was 14%. With the introduction of administration of anti-Rh immunoglobulin [RhIg (rhogam)] within 72 hours of delivering, the incidence of HDFN dropped to 1.5%. Shortly after that, this incidence was further reduced to 0.2% to 0.4% with the use of RhIg during the antenatal period. The half-life of intramuscular RhIg is 21 to 30 days so it should offer coverage for up to 12 weeks. It is estimated that a standard 300 IU dose of RhIg will protect against fetal infusion of 15ml of red cells or 30 ml of whole blood. A smaller 50 IU dose of RhIg (known as min RhIg) is also available for early pregnancy and will protect against 2.5ml of red blood cells or 5ml of whole blood. Since 92% of women will become sensitized after 28 weeks, and for 1-2% this will occur prior to delivery, the most recent guidelines from the American College of Obstetrics and Gynecologists (ACOG) recommends a full dose of 300 IU of RhIg at 28 weeks to be followed by a second dose within 72 hours of delivery. Supplemental dosing is recommended after evidence of any event that with risk of fetal-maternal hemorrhage.
Fetal RBCs can express the Rh(D) antigen as early as 38 days from conception, or 52 days from last menstrual period and fetal-maternal transfusion has been demonstrated as early as 5 to 6 weeks of gestation. Thus, RhIg is recommended in the first trimester in the presence of threatened or complete miscarriage, termination of pregnancy, and ectopic pregnancy. The mean volume of a fetal-maternal transfusion at eight weeks has been calculated to be 0.33 mL. For this reason, mini RhIg should be adequate in the first trimester. For those patients with early, persistent bleeding in the first trimester, mini-RhIg can be given at 6-week intervals.
Once a patient receives RhIg, there is no laboratory test to distinguish between active alloimmunization or passive anti-D antibody (RhIG administration) production. One or the other is assumed by the strength of RBC reaction to agglutinins that test for anti-D antibody. A titer from passive RhIG administration may reach 1:4; a titer ≥ 1:8 requires further investigation for Rh(D) alloimmunization.
Since variability can be seen in performing and reporting a titer result between different labs, the same laboratory should be used to run the titers. A fourfold increase or 2 step change in dilution suggests a true increase in increase in the amount of antibody production that would place the fetus at risk for HDFN. As an example, a titer increase from 1:8 to 1:16 is not significant, but an increase from 1:8 to 1:32 is significant. Each laboratory sets its “critical” titer for anti-D antibody that requires additional action.
The incidence of Rh incompatibility varies by race with approximately 15% of Caucasians are Rh negative compared to 5-8% African Americans. Asians and Native Americans have the lowest prevalence of 1-2%. So if a Rh-negative Caucasian female now mates with a Caucasian male, there is an 85% chance that he will be Rh-positive. This male would have a 60% to 80% chance of being heterozygous for the D antigen. If the partner is heterozygous, the fetus only has 50% chance of being at risk for HDFN yet these patients continue to receive RhIg. It is estimated that approximately 30- 40% of pregnant Rh(-) women carry a Rh(-) fetus and thus upwards of 500,000 women receive RhIg each year unnecessarily.
RhIg is a blood product manufactured in the United States from pooled plasma, predominantly collected from Rh-D negative male plasma donors. These male donors are injected with Rh-D positive red blood cells to stimulate sensitization and antibody production. While the processing and fractionation of plasma is subject to industry standards to minimize the risks of infection or viral transmission or contamination, in the 1970s and 1990s, there were a number of contamination episodes involving RhIg product in countries such as Ireland and Germany. In addition, and what is particularly troubling is that the risks of prion transmission and unknown emerging viruses remains a potential risk for women who continue to receive the product.
Part two on this topic will discuss opportunities to reduce the risk and anxiety of RhIg use. We will explore strategies using new technologies that can identify only those RH(-) women who have a Rh(+) fetus and who truly need RhIg.
Fung Kee, Fung K., Eason, E., Crane, J., Armson, A., De La Ronde, S., Farine, D., Keenan-Lindsay, L., Leduc, L., Reid, G. J., Aerde, J. V., Wilson, R. D., Davies, G., Desilets, V. A., Summers, A., Wyatt, P., and Young, D. C. Prevention of Rh alloimmunization. J Obstet.Gynaecol.Can. 25(9), 765-773. 2003.
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