The Rhesus (Rh) blood group system is the second most important blood group system after ABO, and is central to blood transfusion and pregnancy management. Discovered in 1940, the system currently comprises 56 recognised antigens, though five are most clinically significant: D, C, c, E, and e.
The Rhesus system was named after rhesus monkeys used in the original experiments by Karl Landsteiner and Alexander Wiener in 1940, though the clinical syndrome was first described by Levine and Stetson in 1939. The discovery reshaped transfusion medicine and explained previously unexplained transfusion reactions and cases of newborn jaundice. Today, Rh typing is standard practice in blood banks worldwide.
The D antigen is highly immunogenic, meaning it's likely to cause an immune response if introduced to someone who lacks it. This is why we commonly refer to people as "Rh-positive" (D+) or "Rh-negative" (D-). Identifying the D antigen made it possible to match blood more reliably and to manage Rh incompatibility in pregnancy, preventing many deaths from transfusion reactions and HDFN.
Before the discovery of the Rh system, many transfusions resulted in serious reactions that couldn't be explained by ABO incompatibility. Historical records suggest variable rates of unexplained transfusion reactions, depending on the population and clinical practices of the time. Identifying the Rh factor explained these reactions and later led to the development of RhIg prophylaxis for the prevention of HDFN.
The Rhesus system is encoded by two closely linked genes on chromosome 1: RHD and RHCE. The RHD gene codes for the D antigen, while RHCE codes for the C/c and E/e antigens. These genes are inherited as haplotypes—sets of genetic variations that tend to be inherited together.
Each person inherits one Rh haplotype from each parent. The most common haplotypes in different populations are:
| Haplotype | Fisher-Race | Wiener | Frequency (Caucasian) |
|---|---|---|---|
| Most common D+ | DCe | R1 | 42% |
| Most common D- | dce | r | 39% |
| Second D+ | DcE | R2 | 14% |
| Third D+ | Dce | R0 | 4% |
The combination of these inherited haplotypes determines an individual's Rh phenotype—what we can detect in laboratory tests. Understanding these patterns is essential for predicting inheritance in families and assessing risk in pregnancies.
Understanding Rh blood groups is vital for several critical medical applications:
In transfusion medicine, matching Rh antigens prevents dangerous immune reactions. An Rh-negative person who receives Rh-positive blood may develop antibodies that attack future transfusions. This sensitisation is permanent and can severely limit future transfusion options.
During pregnancy, Rh incompatibility between mother and fetus can lead to Haemolytic Disease of the Fetus and Newborn (HDFN). If an Rh-negative mother carries an Rh-positive baby, her immune system may produce antibodies against the baby's red blood cells. While anti-D is the most common cause, anti-c and anti-E can also cause severe HDFN.
RhIg prophylaxis protocols vary between countries; always follow local guidelines. A commonly used framework includes:
For fetomaternal haemorrhages larger than the standard prophylaxis covers, additional RhIg doses are required. The Kleihauer-Betke test or flow cytometry should be used to quantify the haemorrhage and calculate the appropriate dose. The threshold and calculation method vary by guideline: UK BCSH recommends an additional 500 IU per 4 mL fetal red cells; US guidance uses approximately 10 μg (50 IU) per mL of fetal whole blood. Always refer to your local protocol.
Not all D antigens are created equal. Several important variants require special consideration in clinical practice:
Weak D (formerly "Du") refers to red cells with reduced D antigen expression. These individuals:
Partial D individuals lack portions of the D antigen and can make antibodies against the missing epitopes:
DEL is an extremely weak form of D antigen expression:
Modern blood typing uses various methods to detect Rh antigens on red blood cells. Testing has progressed from manual tube testing to automated platforms and molecular methods.
Immediate Spin: Detects strong D-positive reactions at room temperature
37°C Incubation: Enhances weak reactions through optimal antibody binding
Anti-Human Globulin (AHG): Detects weak D variants and confirms D-negative status
Column Agglutination Technology: Modern automated method providing standardised results
Solid Phase Testing: Used in automated blood bank analysers
Molecular Testing: DNA-based methods for definitive genotyping
Advanced molecular methods can now determine Rh genotypes directly from DNA, particularly useful for:
Rh antigen frequencies vary significantly among ethnic groups, reflecting evolutionary history and population migrations:
| Population | D-negative (%) | Most Common D+ Haplotype | Clinical Implications |
|---|---|---|---|
| Caucasian | 15-17% | DCe (R1) | Highest need for D-negative blood |
| African | 3-5% | Dce (R0) | Higher frequency of variant D and U-negative |
| Asian | <1% | DCe (R1) | Rare D-negative donors, higher DEL frequency |
| Hispanic | 8-10% | DCe (R1) | Intermediate frequencies |
Understanding these patterns helps blood banks maintain appropriate inventory and aids in finding compatible blood for patients with rare phenotypes. It also influences genetic counseling approaches in different populations.
These population differences have practical implications:
The Rhesus system is central to transfusion medicine and prenatal care. The move from serological testing to molecular methods has made it easier to resolve weak D and partial D cases, type patients who are difficult to phenotype, and target RhIg prophylaxis more precisely—all of which reduce the risk of HDFN and transfusion reactions.
↑ Back to Top