The Rhesus (Rh) blood group system is one of the most complex human blood group systems, second only to the ABO system in clinical importance. Discovered in the early 1940s by Karl Landsteiner and Alexander Wiener using the blood of Rhesus macaques, this system determines whether a person's blood is designated as positive or negative.
Unlike the ABO system, where individuals naturally develop antibodies against missing antigens without prior exposure, the Rh system does not naturally produce antibodies. An Rh-negative person will only manufacture Rh antibodies after direct exposure to Rh-positive red blood cells, either through mismatched blood transfusion or during childbirth. This biological feature makes the initial exposure a sensitizing event rather than an immediate reaction.
Rh blood group genetics are controlled by two closely linked genes located on chromosome 1: RHD and RHCE. These genes dictate the presence or absence of specific protein structures on the red blood cell membrane.
Inheritance follows standard Mendelian codominant rules. Parents pass on gene complexes as a single unit, meaning offspring inherit one set of Rh genes from each parent.
While the D antigen gets the most clinical attention, the Rh system actually involves several closely related protein antigens. The principal ones tracked in clinical laboratories are C, c, D, E, and e. Note that there is no 'd' antigen; the lowercase 'd' simply denotes the absence of the D antigen.
These antigens combine to form specific phenotypes on the red blood cell surface. Midwives and laboratory scientists look at these combinations to determine precise compatibility when complex transfusion histories or multiple pregnancies are involved.
The D antigen is by far the most immunogenic of all the non-ABO blood group antigens. This means that if an Rh-negative person receives even a tiny volume of Rh-positive blood, their immune system is extremely likely to mount a vigorous antibody response.
Routine laboratory Rh typing uses anti-D serum to test patient red blood cells. If agglutination (clumping) occurs, the patient is Rh-positive. If no clumping occurs, additional weak-D testing may be performed to ensure a weak D variant is not misclassified as negative, protecting patients and pregnant women from unexpected sensitization.
Rh incompatibility occurs when an Rh-negative mother carries an Rh-positive fetus (typically fathered by an Rh-positive man). During a normal pregnancy, maternal and fetal circulations are kept separate by the placenta. However, microscopic leaks or larger transplacental hemorrhages occur during delivery, miscarriage, ectopic pregnancy, or invasive obstetric procedures.
When fetal red blood cells cross into the maternal circulation, the mother recognizes the D antigen as foreign and produces IgM antibodies initially, quickly followed by class-switched IgG antibodies. Because IgG is small enough to cross the placental barrier, these maternal antibodies enter the fetal circulation in subsequent pregnancies, coating fetal red blood cells and marking them for destruction in the fetal spleen and liver.
Destruction of fetal red blood cells leads to Hemolytic Disease of the Newborn (HDN), also known as erythroblastosis fetalis. Affected fetuses may suffer from severe anemia, generalized edema known as hydrops fetalis, jaundice, and kernicterus due to bilirubin deposition in the brain.
Modern midwifery practice successfully prevents this condition through the administration of Rh immunoglobulin (RhIG). RhIG is given to un-sensitized Rh-negative women at around 28 weeks of gestation and again within 72 hours following the delivery of an Rh-positive baby, as well as after any sensitizing event. RhIG clears fetal cells from the maternal circulation before the mother's immune system can mount its own antibody response.
Ensuring Rh compatibility is a critical safety step in clinical blood transfusion. As a general rule, Rh-negative patients should receive Rh-negative blood, especially women of childbearing age and young girls, to prevent future sensitization and subsequent reproductive complications.
In emergency situations where Rh-negative blood is unavailable, an Rh-negative male or a post-menopausal female may receive Rh-positive blood once, as primary sensitization takes weeks to develop and will not cause an immediate acute hemolytic transfusion reaction. However, repeated transfusions or future pregnancies in women make strict adherence to Rh matching mandatory.