Any of the many blood group antigen systems can lead to isoimmunization, but the number of antigens involved in fetal and neonatal hemolytic disease is limited. The most common antigen involved is part of the Rh (CDE) sys-tem, specifically the D antigen.
The Rh system is a complex of five antigens—including the C, c, D, E, and e antigens—each of which elicits a unique immune response. These antigens are inherited together in distinctive patterns reflecting the underlying genotypic makeup of the parents. C and c are alternate forms of the same antigen, as are E and e, but there is nod antigen. The D antigen is either present or absent. Patientswith the D antigen are termed Rh D-positive, and those lacking this gene, and hence the antigen, are said to be Rh D-negative.Approximately 15% of whites, 5% to 8% of African Americans, and only 1% to 2% of Asians and Native Americans are Rh D-negative.
A variant of the D antigen called the weak D antigen (formerly Du) also exists. If not appropriately diagnosed, patients can be mistakenly classified as Rh D-negative. For thisreason, patients should not be considered Rh D-negative unless efforts have been made to look for the weak D anti-gen. Patients who are Rh weak D-positive should be man-aged the same as those who are Rh D-positive.
Isoimmunization can occur when an Rh D-negative woman is pregnant with a fetus who has inherited the Rh D antigen from its father and is thus Rh D-positive. Any event associated with fetomaternal bleeding can potentially lead to maternal exposure to fetal red blood cells, which can trigger a maternal immuneresponse. These events include:
· Delivery of the placenta
· Threatened, spontaneous, elective, or therapeutic abortion
· Ectopic pregnancy
· Bleeding associated with placenta previa or abruption
· Abdominal trauma
· External cephalic version
The amount of Rh D-positive blood required to cause isoimmunization is small—less than 0.1 mL is sufficient.
One study indicates that 17% of Rh D-negative women who do not receive anti-D immune globulin prophylaxis during pregnancy will become isoimmunized.
As with other antibody-mediated immune responses, the first immunoglobulin (Ig) type produced is of the IgM iso-form, which does not cross the placenta to any extent. The chance of significant fetal or newborn disease in a woman’s first at-risk pregnancy is therefore low. It is, however, important to consider prior pregnancy losses or termina-tions as potential exposures, because they could influence the risk of fetal or newborn disease. In a subsequent preg-nancy, passage of minute amounts of fetal blood across the placenta into the maternal circulation, a relatively common occurrence, can lead to an anamnestic response of mater-nal antibody production, which is more robust and rapid than the initial response.
In the case of some antigens, the mother continues to produce predominantly IgM antibodies that fail to cross the placenta. In other cases, the secondary antibody response is characterized by the production of IgG antibodies that freely cross the placenta, enter the fetal circulation, and bind to antigenic sites on fetal red cells. Red blood cells that are highly bound with antibody are hemolyzed in the fetal reticuloendothelial system and destroyed via complement-mediated pathways. Hemolysis releases bilirubin, and the fetus excretes the bilirubin and its break-down products in urine. If the fetus is able to augment ery-thropoiesis to keep pace with the rate of hemolysis, serious anemia may not develop. However, if large amounts of anti-body cross the placenta resulting in destruction of large numbers of fetal red cells, the fetus may be unable to suffi-ciently replenish the red cells and anemia may ensue.
Typically, the first affected pregnancy is characterized by mild anemia and elevated bilirubin at birth, often neces-sitating treatment for the newborn, such as ultraviolet light and exchange transfusion, as the newborn’s liver may be unable to effectively metabolize and excrete the released bilirubin. Markedly elevated bilirubin levels can lead to ker-nicterus (bilirubin deposition in the basal ganglia) whichcan cause permanent neurologic symptoms or even death. This condition is rarely seen today in developed countries.
In some first-affected pregnancies, and in many, but not all, subsequent pregnancies with an antigen-positive fetus, antibody production increases as a result of the anamnestic response, lead-ing to more significant hemolysis and anemia. Assessment ofthe amount of bilirubin excreted by these fetuses into the amniotic fluid is one method used to monitor fetal status . When fetal anemia is significant, fetal hematopoiesis increases, including the recruitment of alternative sites for red cell production. The fetal liver isan important site of extramedullary hematopoiesis. When the liver produces red blood cells, the production of other proteins decreases, resulting in a lower oncotic pressure within the fetal vasculature. This consequence, in con-junction with the increase in intravascular resistance to flow caused by islands of hematopoietic cells in the liver, can lead to the development of ascites, subcutaneous edema, or pleural effusion.
Severe anemia affects fetal cardiac function in two ways. First, anemia can lead to a high-output cardiac fail-ure. As the cardiac system attempts unsuccessfully to keep pace with the oxygen-delivery demands, the myocardium becomes dysfunctional, resulting in effusions, edema, and ascites due to hydrostatic pressure increases. Second, the anemia itself can cause myocardial ischemia, thereby directly damaging and compromising myocardial func-tion. This combination of fluid accumulation in at least two extravascular compartments (pericardial effusion, pleural effusion, ascites, or subcutaneous edema) is referred to as hydrops fetalis.
Isoimmunization usually progressively worsens in each sub-sequent pregnancy. Fetal anemia may occur at the same ges-tational age or earlier than in the prior affected pregnancies.
Determination of the father’s antigen status is important in assessing whether the fetus is at risk for developing anemia. Any individual can be either homozygous or heterozy-gous for a particular gene. If the father is heterozygous for the gene for the particular antigen of interest, there is a 50% chance that the fetus will not inherit the gene for that antigen. For many of the antigens, this informa-tion can be determined easily by looking at which anti-gens are expressed on the father’s red blood cells. For example, C and c are coded by the same gene, but differ by a single base change. An individual can express C, c, or both. If he expresses both, he is heterozygous; if only one antigen is detected, then he must be homozygous. Unfortunately, the situation is not as straightforward with Rh D (because there is no d antigen). However, direct genotype testing can be performed to determine if the father is homozygous or heterozygous. In a preg-nancy involving an isoimmunized patient, the first step in management is determination of the paternal eryth-rocyte antigen status. In pregnancies in which there is a heterozygous or unknown paternal genotype, the fetal antigen type should be assessed by genetic analysis of fetal cells obtained by amniocentesis.
Regardless of the amount of maternal antibody present, if the subsequent fetus does not carry the antigen (because the father was a heterozygote or there is different paternity), then the fetus has a 98.5% probably of not being at risk.