A PATHOPHYSIOLOGICAL OVERVIEW OF RH INCOMPATIBILITY

Introduction

Rh incompatibility, also known as Rh disease or hemolytic disease of the newborn (HDN), is a condition that occurs when there is an incompatibility between the blood types of a pregnant woman and her fetus. This condition arises when the mother is Rh-negative (lacks the Rh antigen) and the fetus is Rh-positive (expresses the Rh antigen). Rh incompatibility can lead to the production of antibodies by the mother’s immune system against the Rh antigen, which can have significant consequences for both the current and future pregnancies.

The pathophysiology of Rh incompatibility involves a series of immunological reactions that occur in response to the presence of the Rh antigen. When an Rh-negative woman becomes pregnant with an Rh-positive fetus, there is a potential for fetal red blood cells (RBCs) to enter the maternal circulation during pregnancy or childbirth. This exposure to fetal RBCs triggers an immune response in the mother’s body.

The first exposure to fetal RBCs sensitizes the mother’s immune system to produce antibodies against the Rh antigen. This sensitization typically occurs during childbirth or any other event that causes mixing of fetal and maternal blood, such as miscarriage, ectopic pregnancy, or invasive prenatal procedures. The most common antibody produced in response to Rh sensitization is anti-D antibody.

In subsequent pregnancies with an Rh-positive fetus, these maternal antibodies can cross the placenta and enter the fetal circulation. Once in contact with fetal RBCs expressing the Rh antigen, these antibodies bind to and destroy the fetal RBCs through a process called hemolysis. This destruction of fetal RBCs leads to anemia and jaundice in the affected fetus.

The severity of Rh incompatibility can vary depending on several factors, including the amount and potency of maternal antibodies, the gestational age at which sensitization occurs, and whether any interventions are undertaken to prevent or manage the condition. In some cases, Rh incompatibility can result in severe fetal complications, including hydrops fetalis (excessive fluid accumulation in the fetus), fetal heart failure, and even intrauterine death.

To prevent the development of Rh incompatibility, Rh-negative women who are pregnant with an Rh-positive fetus are typically given a medication called Rh immune globulin (RhIg) at around 28 weeks of gestation. RhIg works by neutralizing any fetal RBCs that may have entered the maternal circulation, preventing sensitization and subsequent antibody production. It is also administered within 72 hours after any event that could potentially lead to fetal-maternal blood mixing, such as childbirth or miscarriage.

In conclusion, the pathophysiology of Rh incompatibility involves the sensitization of an Rh-negative mother to the Rh antigen present on fetal RBCs during pregnancy or childbirth. This sensitization leads to the production of antibodies that can cross the placenta and cause hemolysis of fetal RBCs in subsequent pregnancies. Understanding the underlying mechanisms of Rh incompatibility has allowed for the development of preventive measures, such as RhIg administration, to minimize the risks associated with this condition.

 

Five major Rh antigens

The Rh antigen system, also known as the Rhesus blood group system, is one of the most important and complex blood group systems in humans. It consists of more than 50 antigens, but here are five major Rh antigens:

1. D antigen (RhD): The D antigen is the most significant Rh antigen and is responsible for the Rh positive (+) or Rh negative (-) blood types. Individuals who possess the D antigen on their red blood cells are classified as Rh positive, while those lacking the D antigen are considered Rh negative.

2. C antigen (RhC): The C antigen is another important Rh antigen. It is closely related to the D antigen and is often inherited together with it. Individuals who have both the C and D antigens are denoted as Rh positive (C+D+), while those lacking both antigens are classified as Rh negative (C-D-).

3. E antigen (RhE): The E antigen is another significant Rh antigen. Similar to the C antigen, it is often inherited together with the D antigen. Individuals who possess both the E and D antigens are classified as Rh positive (E+D+), while those lacking both antigens are considered Rh negative (E-D-).

4. c antigen (rhc): The c antigen is a minor Rh antigen that is closely related to the C antigen. It can be present in individuals who lack the C antigen, and vice versa. Individuals who possess the c antigen but lack the C antigen are classified as Rh positive (c+D-), while those lacking both antigens are considered Rh negative (c-D-).

5. e antigen (rhe): The e antigen is another minor Rh antigen that is closely related to the E antigen. It can be present in individuals who lack the E antigen, and vice versa. Individuals who possess the e antigen but lack the E antigen are classified as Rh positive (e+D-), while those lacking both antigens are considered Rh negative (e-D-).

These five major Rh antigens play a crucial role in blood transfusion compatibility and in the prevention of hemolytic disease of the newborn. Understanding an individual’s Rh status is essential in determining blood compatibility for transfusions and during pregnancy to prevent complications.

 

Characteristics of hemolytic disease of the newborn (HDN)

Hemolytic disease of the newborn (HDN), also known as erythroblastosis fetalis, is a condition that occurs when there is an incompatibility between the blood types of the mother and the fetus. It is characterized by the destruction of red blood cells (hemolysis) in the fetus or newborn, leading to various complications. HDN typically arises when a mother with Rh-negative blood type carries a fetus with Rh-positive blood type, but it can also occur due to other blood group incompatibilities.

The development of HDN involves a series of events triggered by the mother’s immune response to fetal red blood cells. When an Rh-negative mother is exposed to Rh-positive fetal blood during pregnancy or childbirth, her immune system may produce antibodies against the Rh factor. These antibodies can cross the placenta and attack the red blood cells of the fetus, leading to their destruction.

The characteristics of hemolytic disease of the newborn include:

1. Jaundice: One of the primary signs of HDN is jaundice, which is characterized by yellowing of the skin and eyes. Jaundice occurs due to the accumulation of bilirubin, a yellow pigment produced when red blood cells are broken down. In severe cases, jaundice can lead to complications such as brain damage (kernicterus) if left untreated.

2. Anemia: The destruction of red blood cells in HDN can result in anemia, a condition characterized by a low level of red blood cells or hemoglobin in the bloodstream. Anemia can cause fatigue, pale skin, rapid heartbeat, and shortness of breath in affected newborns.

3. Enlarged liver and spleen: HDN can lead to hepatosplenomegaly, which refers to enlargement of both the liver and spleen. This occurs as these organs work to remove and process the damaged red blood cells. Enlargement of the liver and spleen can cause discomfort and may be detected during a physical examination.

4. Hydrops fetalis: In severe cases of HDN, a condition called hydrops fetalis may develop. Hydrops fetalis is characterized by abnormal accumulation of fluid in the fetus, leading to swelling of the body, including the abdomen, limbs, and organs. This condition can be life-threatening for the fetus.

5. Intrauterine growth restriction: HDN can also result in poor fetal growth due to the destruction of red blood cells and subsequent anemia. This can lead to a smaller-than-average size for gestational age.

6. Neurological complications: In rare cases, severe HDN can cause neurological complications such as seizures, developmental delays, or intellectual disabilities. These complications occur when bilirubin levels become extremely high and cross the blood-brain barrier, leading to brain damage.

Treatment and prevention of HDN involve various approaches depending on the severity of the condition. Mild cases may only require close monitoring of bilirubin levels and phototherapy (light therapy) to help break down excess bilirubin in the baby’s body. In more severe cases, exchange transfusion may be necessary to replace the baby’s blood with compatible donor blood.

To prevent HDN, Rh-negative mothers are typically given an injection of Rh immunoglobulin (RhIg) around 28 weeks of pregnancy and within 72 hours after delivery or any event that may cause mixing of maternal and fetal blood. RhIg helps prevent sensitization of the mother’s immune system to Rh-positive blood cells, reducing the risk of HDN in future pregnancies.

 

Diagnosis of HDN

HDN, or hemolytic disease of the newborn, is a group of disorders that occur when an immune response is triggered against red blood cells (RBCs) in a newborn baby. This can lead to the destruction of RBCs, which can result in anemia, jaundice, and other complications. To establish the diagnosis of HDN, several tests and evaluations are necessary.

A) Clinical Evaluation

The first step in establishing the diagnosis of HDN is a thorough clinical evaluation. This includes:

1. History: A detailed history of the mother’s pregnancy and the newborn’s symptoms, including any bleeding or jaundice.

2. Physical examination: A thorough physical examination of the newborn, including assessment of vital signs, skin color, and presence of any rashes or lesions.

3. Laboratory tests: Laboratory tests to evaluate the newborn’s blood count, liver function, and bilirubin levels.

B) Direct Coombs Test

One of the key tests used to establish the diagnosis of HDN is the direct Coombs test. This test detects the presence of antibodies against RBCs in the newborn’s blood. The direct Coombs test is performed by mixing the newborn’s blood with known RBCs and measuring the reaction between the two. If antibodies are present, they will cause agglutination (clumping) of the RBCs.

C) Indirect Coombs Test

If the direct Coombs test is positive, the indirect Coombs test may also be performed. This test detects the presence of antibodies against RBCs in the mother’s blood. The indirect Coombs test is performed by mixing the mother’s blood with known RBCs and measuring the reaction between the two. If antibodies are present, they will cause agglutination of the RBCs.

D) Blood Smear Examination

A blood smear examination may also be performed to establish the diagnosis of HDN. This involves examining a sample of the newborn’s blood under a microscope for evidence of hemolysis (breakdown of RBCs). Hemolytic anemia, characterized by the presence of nucleated RBCs and fragmented RBCs, is a hallmark of HDN.

E) Liver Function Tests
Liver function tests may also be performed to evaluate the newborn’s liver function. Elevated levels of bilirubin, alanine transaminase (ALT), and aspartate transaminase (AST) are common in HDN.

F) Ultrasound Examination

An ultrasound examination of the liver and spleen may also be performed to evaluate the extent of hemolysis and to rule out other conditions that may cause similar symptoms.

G) Genetic Testing

In some cases, genetic testing may be performed to identify the underlying cause of HDN. This may involve testing for inherited red blood cell antigens, such as the Rh factor, Kell antigens, or other antigens that may be responsible for the development of HDN.

H) Confirmation of Diagnosis

The diagnosis of HDN is confirmed based on a combination of clinical findings, laboratory tests, and imaging studies. The presence of hemolytic anemia, elevated bilirubin levels, and the presence of antibodies against RBCs are all consistent with the diagnosis of HDN.

 

ABO HDN versus Rh HDN

ABO and Rh HDN are both types of hemolytic disease of the newborn (HDN), which is a condition characterized by the destruction of red blood cells in a newborn’s body. However, they differ in terms of the antigens involved and the mechanisms by which they cause hemolysis.

ABO HDN occurs when there is an incompatibility between the blood types of the mother and the baby. The ABO blood group system classifies blood into four types: A, B, AB, and O. These blood types are determined by the presence or absence of specific antigens on the surface of red blood cells. In ABO HDN, if a mother with type O blood (lacking A or B antigens) gives birth to a baby with type A or B blood (expressing either A or B antigens), the mother’s immune system may produce antibodies against the baby’s red blood cells. This can lead to hemolysis and subsequent complications.

On the other hand, Rh HDN is caused by an incompatibility between the Rh factor of the mother and the baby. The Rh factor is another antigen found on red blood cells, and individuals can be either Rh positive (expressing the antigen) or Rh negative (lacking the antigen). If an Rh-negative mother carries an Rh-positive baby, there is a risk that her immune system will produce antibodies against the baby’s red blood cells during pregnancy or childbirth. Subsequent pregnancies with Rh-positive babies can then lead to more severe hemolysis due to the presence of these antibodies.

The mechanisms underlying ABO and Rh HDN also differ. In ABO HDN, antibodies against A or B antigens are typically of the IgM class, which cannot cross the placenta easily. Therefore, ABO HDN is usually milder and less severe than Rh HDN. In contrast, Rh HDN involves antibodies of the IgG class, which can cross the placenta and directly attack the baby’s red blood cells. This can result in more severe hemolysis and potentially life-threatening complications.

To diagnose ABO and Rh HDN, blood tests are performed to determine the blood types of both the mother and the baby. In cases of ABO HDN, the direct Coombs test may be used to detect antibodies on the baby’s red blood cells. In Rh HDN, the indirect Coombs test is commonly employed to detect the presence of maternal antibodies in the mother’s blood.

Treatment for both ABO and Rh HDN aims to manage the complications associated with hemolysis. This may include phototherapy to treat jaundice, blood transfusions to replace damaged red blood cells, and close monitoring of the baby’s condition. In severe cases, exchange transfusions may be necessary to remove the antibodies from the baby’s bloodstream.

In summary, ABO HDN occurs due to an incompatibility between the ABO blood types of the mother and baby, while Rh HDN arises from an incompatibility between the Rh factors. ABO HDN is usually milder and involves IgM antibodies, while Rh HDN is more severe and involves IgG antibodies that can cross the placenta. Proper diagnosis and management are crucial in both conditions to ensure the well-being of the newborn.

 

Tests used for detection of Fetomaternal hemorrhage (FMH)

Fetomaternal hemorrhage (FMH) refers to the transfer of fetal blood into the maternal circulation during pregnancy or childbirth. It can occur spontaneously or as a result of trauma, invasive procedures, or certain medical conditions. Detecting FMH is crucial as it can lead to complications such as fetal anemia, isoimmunization, and subsequent hemolytic disease of the newborn. Several tests are available for the detection of FMH, which can vary in sensitivity and specificity.

1. Kleihauer-Betke (KB) test: The KB test is one of the most commonly used tests for detecting FMH. It is based on the principle that fetal red blood cells (RBCs) contain fetal hemoglobin (HbF), which is resistant to acid elution, while maternal RBCs contain adult hemoglobin (HbA), which is susceptible to acid elution. In this test, a maternal blood sample is mixed with an acid solution to elute maternal HbA, leaving behind fetal HbF-containing RBCs. The remaining fetal RBCs are then stained and counted under a microscope. The percentage of fetal RBCs can be determined by comparing them to a known standard. This test is particularly useful for estimating the volume of FMH.

2. Flow cytometry: Flow cytometry is a more sensitive and specific method for detecting FMH compared to the KB test. It involves labeling maternal RBCs with a fluorescent dye that binds specifically to fetal hemoglobin. The labeled cells are then analyzed using flow cytometry, which can accurately quantify the percentage of fetal RBCs in the maternal circulation. This method allows for the detection of even small volumes of FMH.

3. Rosette test: The rosette test is another commonly used test for detecting FMH. It relies on the fact that fetal RBCs express different antigens on their surface compared to maternal RBCs. In this test, a maternal blood sample is mixed with a reagent containing antibodies specific to fetal RBC antigens. If FMH has occurred, the fetal RBCs will form rosettes (clusters) around the antibody-coated particles. The number of rosettes can be counted under a microscope, providing an estimate of the volume of FMH.

4. Dithiothreitol (DTT) test: The DTT test is based on the principle that fetal RBCs are more resistant to DTT-induced hemolysis compared to maternal RBCs. In this test, a maternal blood sample is treated with DTT, which selectively lyses maternal RBCs while leaving fetal RBCs intact. The remaining fetal RBCs can then be counted and compared to a known standard to estimate the volume of FMH.

5. Fetal cell-free DNA (cfDNA) analysis: Recent advancements in molecular techniques have led to the development of non-invasive methods for detecting FMH using cfDNA analysis. During pregnancy, small amounts of fetal DNA circulate in the maternal bloodstream. By analyzing the relative amounts of fetal and maternal DNA, it is possible to estimate the volume of FMH. This method is particularly useful for detecting FMH in Rh-negative mothers at risk of isoimmunization.

It is important to note that the choice of test for detecting FMH may depend on various factors such as availability, cost, and clinical context. Additionally, some tests may require specialized laboratory equipment or expertise.

 

The blood group to be used in HDN

Hemolytic Disease of the Newborn (HDN), also known as erythroblastosis fetalis, is a condition that occurs when there is an incompatibility between the blood types of the mother and the fetus. This condition can lead to the destruction of fetal red blood cells, resulting in various complications.

To determine the blood group to be used in HDN, it is essential to understand the concept of blood typing and its relevance in this condition. Blood typing involves identifying specific antigens present on the surface of red blood cells. The two most important blood group systems involved in HDN are the ABO system and the Rh system.

In the ABO system, there are four main blood types: A, B, AB, and O. The presence or absence of specific antigens (A and B) on the surface of red blood cells determines these blood types. In HDN, if a mother with type O blood (lacking both A and B antigens) carries a fetus with type A or B blood (possessing either A or B antigen), there is a risk of developing HDN. This occurs because the mother’s immune system recognizes the fetal red blood cells as foreign and produces antibodies against them.

The Rh system is another crucial factor in HDN. The Rh antigen, also known as the D antigen, is either present (Rh positive) or absent (Rh negative) on red blood cells. If an Rh-negative mother carries an Rh-positive fetus, there is a potential for sensitization during pregnancy or delivery. Sensitization occurs when maternal Rh-negative blood comes into contact with fetal Rh-positive blood, leading to the production of antibodies against the Rh antigen. In subsequent pregnancies with Rh-positive fetuses, these antibodies can cross the placenta and cause hemolysis of fetal red blood cells.

To prevent or manage HDN, various interventions can be employed depending on the severity of the condition. One of the primary strategies is the administration of Rh immune globulin (RhIg) to Rh-negative mothers. RhIg is a blood product that contains antibodies against the Rh antigen. It is given to Rh-negative mothers at specific times during pregnancy and after delivery to prevent sensitization and subsequent HDN in future pregnancies.

In cases where HDN has already developed, blood transfusions may be necessary to replace the destroyed red blood cells in the fetus or newborn. The choice of blood group for transfusion depends on several factors, including the severity of HDN, the availability of compatible blood, and the presence of other complicating factors.

When selecting blood for transfusion in HDN, it is crucial to consider both the ABO and Rh compatibility between the donor and recipient. For ABO compatibility, it is generally recommended to use blood from donors with the same ABO type as the recipient. For example, if the newborn has type A blood, it is preferable to transfuse type A blood. However, in emergency situations or when compatible blood is not readily available, type O negative (O-) blood can be used as a universal donor for both A and B types.

Regarding Rh compatibility, it is essential to avoid transfusing Rh-positive blood to an Rh-negative recipient to prevent sensitization and further complications. Therefore, in cases where the mother is Rh-negative and the newborn requires a blood transfusion, it is crucial to use Rh-negative blood.

In summary, when considering the blood group to be used in HDN, it is important to ensure both ABO and Rh compatibility between the donor and recipient. ABO compatibility should prioritize using blood from donors with the same ABO type as the recipient whenever possible. However, in emergency situations or when compatible blood is not readily available, type O negative (O-) blood can be used as a universal donor for both A and B types. Additionally, Rh-negative blood should be used when the mother is Rh-negative to prevent sensitization and further complications.