# Alloimmunization and Rh Disease

## Overview

### Definition

Red cell alloimmunization occurs when a pregnant woman's immune system produces IgG antibodies directed against fetal red blood cell antigens inherited from the father. These IgG antibodies cross the placenta and can cause fetal hemolytic anemia, hydrops fetalis, and fetal death -- collectively termed hemolytic disease of the fetus and newborn (HDFN). While the Rh(D) antigen is the most commonly involved, alloimmunization can occur against many different red cell antigens.

### Epidemiology

Rh(D) negativity is present in approximately 15% of White populations, 5 to 8% of Black and Hispanic populations, and less than 1% of Asian populations. Before the introduction of anti-D immunoglobulin (RhoGAM), sensitization occurred in approximately 16% of Rh(D)-negative women carrying Rh(D)-positive fetuses. With modern prophylaxis, the sensitization rate has been reduced to approximately 0.1 to 0.2%, making RhoGAM one of the most successful preventive interventions in obstetrics.

## Rh(D) Alloimmunization

### Mechanism

When an Rh(D)-negative mother is exposed to Rh(D)-positive fetal red blood cells during fetomaternal hemorrhage, she mounts an initial primary immune response producing IgM antibodies, which are too large to cross the placenta. However, upon subsequent exposure -- in the same or a future pregnancy -- an anamnestic IgG response occurs. These IgG antibodies cross the placenta and bind to fetal red blood cells, targeting them for destruction. The resulting fetal hemolysis leads to anemia, compensatory extramedullary hematopoiesis (in the liver and spleen), and in severe cases, hydrops fetalis.

### Sensitizing Events

Fetomaternal hemorrhage can occur during delivery (the most common sensitizing event), miscarriage, ectopic pregnancy, induced abortion, amniocentesis, chorionic villus sampling, cordocentesis, external cephalic version, placental abruption, abdominal trauma, and manual removal of the placenta. Spontaneous or silent fetomaternal hemorrhage can also occur without an identifiable precipitating event.

### Prevention: Anti-D Immunoglobulin (RhoGAM)

Standard prophylaxis consists of 300 mcg of anti-D immunoglobulin IM at 28 weeks' gestation and again within 72 hours of delivery of an Rh(D)-positive infant. Additional doses are given after any sensitizing event: 50 mcg (or 300 mcg, as both are acceptable) for first-trimester events, and 300 mcg for second- or third-trimester events. The Kleihauer-Betke (KB) test or flow cytometry is used to quantify the volume of fetomaternal hemorrhage. A standard dose of RhoGAM covers 30 mL of fetal whole blood (15 mL of fetal red blood cells). If the KB test indicates more than 30 mL of fetal blood, additional doses are needed, calculated as the volume of fetal blood divided by 30, rounded up, with one additional vial added for safety.

<image>Flowchart for Rh(D) immunoglobulin prophylaxis in pregnancy, showing decision points at booking (antibody screen), 28 weeks (routine prophylaxis), sensitizing events, and postpartum administration based on newborn Rh status and Kleihauer-Betke results</image>

## Non-D Alloimmunization

### Clinically Significant Antibodies

Anti-Kell (anti-K) is the second most common cause of severe HDFN. It has a unique mechanism: rather than simply causing hemolysis, anti-Kell antibodies suppress fetal erythropoiesis, meaning fetal anemia can be more severe than antibody titers suggest. MCA Doppler surveillance is therefore essential, and titers alone should not be relied upon to assess disease severity. Anti-c, anti-E, and anti-C can all cause significant HDFN. Anti-Duffy (Fya) and anti-Kidd (Jka/Jkb) are less common but capable of causing severe disease. Lewis and I antibodies are IgM class, do not cross the placenta, and are not clinically significant for HDFN.

| Antibody | Severity | Mechanism | Critical Titer | Key Point |
|---|---|---|---|---|
| Anti-D | Severe HDFN | Hemolysis | 1:16 (some use 1:8) | Most common; preventable with RhoGAM |
| Anti-Kell | Severe HDFN | Suppresses erythropoiesis | 1:8 or any positive | Anemia worse than titers predict |
| Anti-c | Moderate-severe | Hemolysis | 1:16 | Third most common cause |
| Anti-E | Mild-moderate | Hemolysis | 1:16 | Usually mild |
| Anti-Duffy (Fya) | Variable | Hemolysis | 1:16 | Less common |
| Anti-Kidd (Jka/Jkb) | Variable | Hemolysis | 1:16 | Less common |
| Lewis, I | Not significant | IgM (does not cross placenta) | N/A | No antenatal surveillance needed |

### ABO Incompatibility

ABO incompatibility is the most common cause of hemolytic disease in the newborn but is typically mild. It rarely causes fetal anemia severe enough to require in utero intervention and usually presents as neonatal jaundice requiring phototherapy. It does not require antenatal surveillance.

## Evaluation of the Sensitized Patient

### Antibody Titer Monitoring

An antibody screen is performed at the first prenatal visit. When a clinically significant antibody is identified, the titer is obtained by indirect Coombs test. The critical titer -- the threshold above which fetal anemia becomes a concern -- is generally 1:16 for anti-D (though some institutions use 1:8). For anti-Kell, the critical titer is lower, at 1:8 or even any positive titer at some centers. Below the critical titer, titers are repeated monthly. Once the critical titer is reached or exceeded, MCA Doppler surveillance is initiated, and further titer measurements become unreliable predictors of disease severity.

### Paternal and Fetal Antigen Testing

Determining the father's antigen status can clarify fetal risk. If the father is antigen-negative, the fetus cannot be affected (assuming confirmed paternity). If the father is heterozygous, there is a 50% chance the fetus is antigen-positive. If homozygous, the fetus is certainly antigen-positive. Cell-free fetal DNA (cffDNA) testing can now determine fetal Rh(D) status non-invasively from a maternal blood sample with greater than 99% sensitivity. If the fetus is found to be Rh(D)-negative, no further monitoring is needed. This technology is increasingly available for other antigens including Kell, c, and E.

## Fetal Surveillance: MCA Doppler

### Rationale

Fetal anemia decreases blood viscosity and triggers an increase in cardiac output, both of which result in increased flow velocity in the middle cerebral artery. MCA peak systolic velocity (PSV) therefore correlates with the degree of fetal anemia.

### Technique

Using color Doppler, the MCA is identified near its origin from the circle of Willis. The PSV is measured with the angle of insonation kept as close to 0 degrees as possible. Values are plotted on gestational age-specific nomograms (Mari curves). An MCA PSV at or above 1.5 multiples of the median (MoM) is suggestive of moderate to severe fetal anemia, with approximately 100% sensitivity and a false-positive rate of about 12%.

### Monitoring Schedule

MCA Doppler monitoring begins at 24 weeks (or earlier if a prior pregnancy was severely affected) and is repeated every 1 to 2 weeks depending on disease severity. The reliability of MCA PSV decreases after 35 weeks due to a higher false-positive rate, and delivery may be preferred over continued invasive testing at that gestational age.

<image>Doppler ultrasound image of the fetal middle cerebral artery showing peak systolic velocity measurement, with a reference nomogram overlay indicating the 1.5 MoM threshold for moderate-to-severe fetal anemia</image>

## Intrauterine Transfusion (IUT)

### Indications

Intrauterine transfusion is indicated when the MCA PSV is at or above 1.5 MoM, suggesting moderate to severe fetal anemia, or when fetal hydrops is present (ascites, pleural effusion, skin edema, polyhydramnios). It is typically performed between 18 and 35 weeks' gestation.

### Technique

The procedure is performed by a maternal-fetal medicine specialist under continuous ultrasound guidance. The preferred approach is intravascular transfusion, in which a needle is inserted into the umbilical vein at the cord insertion site or intrahepatic vein. Fetal blood is aspirated to confirm venous placement and measure the fetal hematocrit. O-negative, CMV-negative, irradiated, leukoreduced, and tightly crossmatched packed red blood cells are infused, targeting a post-transfusion hematocrit of 40 to 45%. Intraperitoneal transfusion is an alternative when intravascular access is difficult, though absorption is slower.

### Complications

The most common complication is fetal bradycardia, which is usually transient. Other complications include PPROM, infection, fetal death (approximately 1 to 3% per procedure), and cord tamponade or hemorrhage.

### Monitoring After IUT

Serial MCA Doppler continues after IUT, though measurements become less reliable because the fetal blood is predominantly donor adult red blood cells. Repeat IUT is typically needed every 2 to 4 weeks based on the predicted rate of hematocrit decline. Delivery is usually planned at 37 to 38 weeks, or earlier if the interval between transfusions suggests a rapidly declining hematocrit.

## Delivery and Neonatal Management

### Timing

Delivery timing depends on disease severity and gestational age. Mild disease that did not require IUT is typically delivered at 37 to 38 weeks. Severe disease requiring multiple IUTs is often delivered at 34 to 37 weeks, balancing the ongoing risk of fetal anemia against the risks of prematurity.

### Neonatal Considerations

Cord blood is sent for a direct Coombs test. The neonate's hematocrit, bilirubin, and reticulocyte count are monitored. Phototherapy is used for hyperbilirubinemia, with exchange transfusion reserved for severe cases. Late-onset anemia may develop weeks after birth because the maternal antibodies (particularly anti-D and anti-Kell) can suppress fetal bone marrow erythropoiesis. Infants who received IUT may have a suppressed reticulocyte count and delayed endogenous red blood cell production, requiring ongoing hematologic monitoring.

## Clinical Pearls

RhoGAM is one of the most successful preventive interventions in obstetrics. It must be administered at 28 weeks and after every sensitizing event without exception.

Anti-Kell antibodies suppress erythropoiesis rather than simply causing hemolysis, so fetal anemia can be more severe than titers predict. MCA Doppler should always be used for surveillance rather than relying on titers alone.

An MCA PSV at or above 1.5 MoM has approximately 100% sensitivity for moderate to severe fetal anemia. A normal MCA PSV is highly reassuring.

Cell-free fetal DNA for Rh(D) typing can eliminate unnecessary surveillance in pregnancies where the fetus is determined to be antigen-negative.

ABO incompatibility almost never causes in utero anemia severe enough to require intervention. It is primarily a neonatal issue managed with phototherapy.

Lewis and I antibodies are IgM class and clinically insignificant for HDFN. Their identification on an antibody screen should not prompt antenatal surveillance.

After 35 weeks, MCA Doppler has a higher false-positive rate. Consider delivery rather than pursuing additional invasive testing at that gestational age.

## References

- ACOG Practice Bulletin No. 192: Management of Alloimmunization During Pregnancy (2018)
- Mari G et al. Noninvasive diagnosis by Doppler ultrasonography of fetal anemia due to maternal red-cell alloimmunization. N Engl J Med. 2000;342:9-14
- Moise KJ Jr. Management of rhesus alloimmunization in pregnancy. Obstet Gynecol. 2008;112:164-176
- SMFM Consult Series No. 50: Red Cell Alloimmunization in Pregnancy (2020)
- De Haas M et al. Anti-D prophylaxis: past, present, and future. Transfus Med Rev. 2014;28:1-7