Genetic basis of the Rh system
The inheritance patterns of the Rhesus factor follow the rules of independent assortment. This means that the genes responsible for this trait are located on different (non-homologous) chromosomes relative to other systems, or are located far apart on the same chromosome. A classic example of such independent assortment in humans is the separate inheritance of ABO blood groups and Rhesus status.
At the genetic level, the synthesis of Rh system antigens is controlled by a complex of three pairs of closely linked non-allelic genes, designated by the Latin letters Cc, Dd, and Ee. These genes are localized on the short arm of chromosome 1. Due to various combinations of these genes, 27 different genotypes are possible in the population.
A key role in determining status is played by the D gene, which is found in the vast majority (85%) of people. Inheritance follows a dominant pattern:
- Rhesus-positive status (Rh+) is established when at least one dominant allele is present — genotypes DD (homozygous) or Dd (heterozygous).
- Rhesus-negative status (Rh-) manifests exclusively in the homozygous recessive state — genotype dd.
Etiology and pathogenesis of Rhesus incompatibility
An immunological conflict between mother and fetus occurs under strictly defined conditions: the pregnant woman must have Rhesus-negative blood, the father must have Rhesus-positive blood (in a homozygous state), and the fetus must inherit the positive Rhesus factor (Rh+).
The development of the pathology has distinct stages and depends on the parity of the pregnancy:
- First pregnancy. Usually proceeds without complications. The blood circulation of the mother and the developing fetus is securely separated by the placental barrier, so foreign fetal erythrocytes do not enter the maternal organism throughout the gestation period. However, during delivery, at the time of placental detachment, a small amount of fetal blood (literally a few milliliters) inevitably enters the woman's bloodstream. This leads to primary sensitization — the maternal immune system recognizes the foreign D antigen and begins actively producing specific anti-D antibodies.
- Subsequent pregnancy. If the second fetus also inherits Rh+, a serious threat arises. Anti-D IgG antibodies accumulated in the mother's body can freely cross the placental barrier. Entering the fetal bloodstream, they massively attack its erythrocytes, triggering a severe immunological conflict. The result of this process is massive fetal erythrocyte hemolysis.
Consequences of incompatibility and treatment approaches
The destruction of red blood cells leads to catastrophic consequences for the developing organism. In severe cases, Rhesus incompatibility can result in spontaneous miscarriage or stillbirth.
If the child is born alive, hemolytic disease of the newborn (HDN) is diagnosed. The main clinical manifestations of this life-threatening condition are rapidly progressive hemolytic jaundice and severe anemia. To save the life of such a newborn, emergency medical intervention is required — an urgent transfusion of Rhesus-negative blood, which stops erythrocyte destruction and restores the oxygen-carrying capacity of the blood.
Specific immunoprophylaxis
To prevent the development of sensitization and protect future children, the Finn method, proposed by Soviet microbiologist E.R. Finn in 1961, is used. The implementation of this technique has reduced child mortality rates from hemolytic disease of the newborn from 10% to zero.
The essence of prophylaxis is the intramuscular administration of specific anti-D antibodies (immunoglobulin) to the woman. The medication must be administered strictly within the first 72 hours after delivery or abortion. The standard dosage is 250–300 mcg.
Mechanism of action: The administered dose of immunoglobulin can rapidly bind and neutralize about 30 ml of fetal erythrocytes that managed to enter the maternal circulation during placental detachment. As a result, the woman's immune system does not have time to trigger active immunization and does not produce its own aggressive antibodies.
Important clinical rule: Rhesus-negative women are strictly prohibited from receiving Rhesus-positive blood transfusions during any medical procedures. Ignoring this rule will inevitably lead to profound sensitization of the organism, which in the future threatens severe pregnancy loss or permanent infertility.
Briefly: answers to topical questions
What are the clinical forms of hemolytic disease of the newborn (HDN)?
Hemolytic disease of the newborn has three clinical forms:
- Hemolytic anemia without jaundice or hydrops — the mildest form; pallor of the skin is noted at birth.
- Hemolytic jaundice — the most common form; jaundice appears at birth or in the first hours of life.
- Congenital fetal hydrops — the most severe form; accompanied by pronounced pallor and generalized edema with fluid accumulation in the pleural, pericardial, and abdominal cavities.
What classes of antibodies are produced by the mother during primary and secondary sensitization to the Rhesus factor?
During Rhesus incompatibility, the mother produces anti-D antibodies of different classes depending on the stage of sensitization.
- Primary sensitization — when fetal erythrocytes enter the maternal circulation, anti-D antibodies, predominantly IgM, are produced. They practically do not cross the placenta, but they form memory cells.
- Subsequent pregnancies / re-immunization — large amounts of IgG, especially IgG3, against the D antigen are produced. These antibodies cross the placenta and cause fetal erythrocyte hemolysis.
What are the indications for intrauterine fetal blood transfusion in Rhesus incompatibility?
Intrauterine fetal blood transfusion is indicated in severe forms of fetal hemolytic disease and signs of severe anemia.
Situations confirmed by sources include:
- MCA-PSV ≥ 1.5 MoM — high probability of severe anemia; repeat ultrasound is performed in 2–3 days.
- Rising MCA-PSV — cordocentesis followed by intrauterine blood transfusion is indicated.
- Detection of the hydropic form of HDN in the fetus — cordocentesis is performed to accurately determine the fetal hemoglobin level; in severe cases, the main method of correction is intrauterine transfusion.
Intrauterine intravascular transfusions of washed erythrocytes to the fetus are the main method of treatment for severe forms of fetal hemolytic disease.
To remember the timing of prophylaxis: "Three days to save the future" — immunoglobulin is administered strictly within 72 hours (3 days) after pregnancy termination.
Class and exam questions
How many genotypes are possible in the Rh system?
In the Rh factor system, due to the combination of three pairs of closely linked non-allelic genes (Cc, Dd, Ee), 27 different genotypes can be formed.
How does the administered immunoglobulin work (Finn method)?
A dose of 250–300 mcg of anti-D antibodies binds and destroys up to 30 ml of Rhesus-positive fetal blood that has entered the mother before her own immune system recognizes the antigen and begins active production of its own antibodies.
What else to study on this topic
- Interaction of the ABO system and the Rhesus factor
- Pathophysiology of hemolysis in hemolytic disease of the newborn
- Placental barrier permeability for various classes of immunoglobulins
- Genetics of closely linked genes and crossover frequency
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