Summary of "Hemolytic Disease of the Newborn EXPLAINED | Rh and ABO incompatibility"

Core topic

Hemolytic disease of the newborn (HDN) is an alloimmune hemolytic anemia in which maternal antibodies against fetal red blood cell (RBC) antigens cross the placenta and destroy fetal RBCs.

Key antigens and blood-group systems

Two clinically important incompatibilities that cause HDN:
- Rh (D) incompatibility — non-ABO (Rh) antigen on RBCs.
- ABO incompatibility — A and B antigens.
Other minor blood-group incompatibilities exist but are less common and are not covered here.

Rh (D) incompatibility — mechanism and important facts

Basics and inheritance

Rh positive = D antigen present on RBCs; Rh negative = D antigen absent.
If both parents are Rh negative → child always Rh negative.
If mother is Rh negative and father Rh positive → child may be Rh positive.

Maternal sensitization

An Rh-negative mother exposed to fetal Rh-positive RBCs can form anti-D antibodies. Sensitizing events include:
- Fetomaternal hemorrhage during childbirth
- Trauma, placental abruption
- Invasive procedures (amniocentesis, chorionic villus sampling)
- Blood transfusion with Rh-positive blood

Antibodies and timing

Primary immune response often produces IgM (does not cross the placenta) → HDN usually not severe in first pregnancy.
IgG antibodies cross the placenta and cause fetal hemolysis; they typically form after sensitization, so HDN classically occurs in subsequent pregnancies.
Clinical severity: Rh incompatibility is less common than ABO incompatibility but usually more severe.

Prevention

Rho(D) immune globulin (RhoGAM) is given to Rh-negative mothers (commonly at ~28 weeks and after delivery or any sensitizing event) to prevent formation of maternal anti-D antibodies.

ABO incompatibility — mechanism and important facts

Blood types (genotype → phenotype)

Phenotype A = AA or AO
Phenotype B = BB or BO
Phenotype AB = AB
Phenotype O = OO

Mother–fetus ABO incompatibility

Most commonly occurs when the mother is type O and the fetus is type A, B, or AB; the mother has anti-A and anti-B antibodies.
Anti-A/anti-B antibodies often develop naturally in early life from environmental antigen exposure.

Antibodies, effect, and clinical course

Many anti-A/anti-B antibodies are IgM (poorly cross the placenta) → ABO HDN is usually milder.
Neonatal RBCs express A and B antigens weakly, reducing severity.
A/B antigens are also on other tissues, which lessens RBC-specific destruction.
ABO HDN can occur in the first pregnancy because antibodies may be pre-existing.
Overall, ABO incompatibility is more common than Rh incompatibility but usually less severe.

Pathophysiology (sequence)

Fetus expresses paternal-derived blood-group antigens on RBCs.
Fetal RBCs or antigens enter the maternal circulation.
Maternal immune system recognizes the antigen as foreign and produces antibodies (in sensitized mothers these are IgG).
Maternal IgG crosses the placenta, binds fetal RBC antigens → hemolysis → fetal anemia, hyperbilirubinemia, and in severe cases hydrops fetalis.

Prevention and clinical implications

Rh: prophylactic RhoGAM for Rh-negative mothers prevents sensitization.
ABO: prevention is less predictable because antibodies are often naturally present; ABO HDN is generally milder and managed supportively.
Monitoring of at-risk pregnancies includes maternal antibody titers, fetal anemia assessment (Doppler measurement of middle cerebral artery peak systolic velocity), and possible intrauterine transfusion for severe fetal anemia.
Neonatal treatments: phototherapy, exchange transfusion, and other supportive care as needed.

Concise comparison: Rh vs ABO incompatibility

Frequency: ABO incompatibility more common; Rh incompatibility less common.
Severity: Rh > ABO (Rh usually more severe).
Pregnancy timing: Rh HDN typically worse in second/subsequent pregnancies (time required to form IgG) unless prior sensitization. ABO HDN can occur in the first pregnancy.
Antibody class: Rh-related harmful antibodies are IgG. ABO antibodies are often IgM (less placental transfer), though some IgG anti-A/B can exist.

Clinical clues and laboratory findings

Maternal IgG antibodies detected against fetal antigens → risk for fetal RBC hemolysis.
Suspect incompatibility when a neonate has hemolysis, anemia, and jaundice and maternal antibodies match fetal antigens.
Severity correlates with high maternal IgG titers and presence of Rh incompatibility.

Practice question (teaching point)

One-day-old neonate with HDN; both parents Rh positive; maternal IgG present. Because both parents are Rh positive → Rh incompatibility is unlikely. The likely cause is ABO incompatibility, specifically mother type O and father type AB (mother O has anti-A/B; father AB can give A or B allele).

Definitions and quick facts

Antigen: substance that induces antibody formation.
Alloimmune hemolytic anemia: immune response against non-self antigens from the same species (maternal vs fetal).
RhoGAM administration around 28 weeks and postpartum reduces the risk of Rh sensitization.
ABO HDN: mother type O can have natural anti-A and anti-B → can present in first pregnancy and is usually mild.