Hemolytic Disease of the Newborn: ABO and Rh Incompatibility

Vocabulary flashcards covering the pathophysiology, clinical differences, complications, and diagnostic testing (Direct and Indirect Coombs tests) of ABO and Rh incompatibility in Hemolytic Disease of the Newborn.

ABO Incompatibility in Hemolytic Disease of the Newborn

A generally mild condition that classically occurs when a blood group O mother carries a blood group A or B infant, resulting from maternal anti-A or anti-B IgG antibodies crossing the placenta and targeting fetal red blood cells.

Sensitization in ABO Incompatibility

The maternal immune priming process that is not required in ABO incompatibility because the mother already possesses pre-existing anti-A and anti-B antibodies, allowing the first infant to potentially be affected.

IgG vs. IgM in Transplacental Transfer

IgM antibodies are pentameric and too large to cross the placenta, whereas the smaller fraction of monomeric IgG antibodies can freely cross the placenta to bind fetal red blood cell antigens.

Rh Incompatibility

A generally severe condition occurring when an Rh-negative mother carries an Rh-positive fetus, leading to maternal alloimmunization against fetal Rh red blood cell antigens and subsequent hemolysis in future Rh-positive pregnancies.

D Antigen (RhD)

The minor red blood cell surface antigen present in Rh-positive individuals that is recognized as foreign by an Rh-negative mother, eliciting an immune response upon exposure.

Sensitization in Rh Incompatibility

The required initial maternal exposure to Rh-positive fetal red blood cells—typically during placental delivery or fetal-maternal hemorrhage—that stimulates antigen-presenting cells, T cells, and B cell class switching to form IgG memory antibodies.

Clinical Status of the First Rh-Positive Infant

Typically asymptomatic because maternal blood mixing and subsequent antibody generation/class switching occur late in pregnancy or during delivery, leaving inadequate time for transplacental antibody destruction.

Clinical Risk for the Second Rh-Positive Infant

A high risk for severe hemolytic disease because the sensitized mother already possesses anti-D IgG antibodies that cross the placenta to target and destroy the infant's Rh-positive erythrocytes.

Hydrops Fetalis

A severe, fluid-overloaded condition caused by extensive red blood cell destruction leading to fetal hypoxemia, activation of the renin-angiotensin system, elevated fetal central venous pressure, circulatory problems, and massive edema.

Kernicterus

Potentially irreversible central nervous system damage resulting from excessive unconjugated (indirect) bilirubin crossing and depositing in brain tissue following massive red blood cell destruction.

Indirect Coombs Test

A laboratory screening test using maternal serum mixed with known Rh-positive donor red blood cells and Coombs reagent (anti-globulin) to detect circulating, unbound maternal anti-D antibodies.

Direct Coombs Test

A diagnostic test performed directly on infant or fetal blood mixed with Coombs reagent (anti-globulin) to detect maternal anti-D antibodies already bound to the infant's red blood cell surface antigens.

Coombs Reagent (Anti-Globulin)

An antibody reagent that binds to the Fc portion of IgG antibodies bound to red blood cells, cross-linking them to produce visible agglutination.