Rh blood group system
The Rh blood group system is a human blood group system. It is the second most important blood group system, after the ABO blood group system. The Rh blood group system consists of 49 defined blood group antigens, among which the five antigens D, C, c, E, and e are the most important. There is no d antigen. Rh status of an individual is normally described with a positive or negative suffix after the ABO type antigen, whereas someone who is A Negative lacks the Rh. The terms Rh factor, Rh positive, and Rh negative refer to the Rh antigen only. Antibodies to Rh antigens can be involved in hemolytic transfusion reactions and antibodies to the Rh and Rh antigens confer significant risk of hemolytic disease of the fetus and newborn.
The term "Rh" was originally an abbreviation of "Rhesus factor." It was discovered in 1937 by Karl Landsteiner and Alexander S. Wiener, who, at the time, believed it to be a similar antigen found in rhesus macaque red blood cells. It was subsequently learned the human factor is not identical to the rhesus monkey factor, but by then, "Rhesus Group" and like terms were already in widespread, worldwide use. Thus, notwithstanding it is a misnomer, the term survives. Contemporary practice is to use "Rh" as a term of art instead of "Rhesus".
The significance of their discovery was not immediately apparent and was only realized in 1940, after subsequent findings by Philip Levine and Rufus Stetson. The serum that led to the discovery was produced by immunizing rabbits with red blood cells from a rhesus macaque. The antigen that induced this immunization was designated by them as Rh factor to indicate that rhesus blood had been used for the production of the serum.
In 1939, Phillip Levine and Rufus Stetson published in a first case report the clinical consequences of non-recognized Rh factor, hemolytic transfusion reaction, and hemolytic disease of the newborn in its most severe form. It was recognized that the serum of the reported woman agglutinated with red blood cells of about 80% of the people although the then known blood groups, in particular ABO were matched. No name was given to this agglutinin when described. In 1940, Karl Landsteiner and Alexander S. Wiener made the connection to their earlier discovery, reporting a serum that also reacted with about 85% of different human red blood cells.
In 1941, Group O: a patient of Dr. Paul in Irvington, NJ, delivered a normal infant in 1931: this pregnancy was followed by a long period of sterility. The second pregnancy resulted in an infant suffering icterus gravis. In May 1941, the third anti-Rh serum of Group O became available.
Based on the serologic similarities, Rh factor was later also used for antigens, and anti-Rh for antibodies, found in humans such as those previously described by Levine and Stetson. Although differences between these two sera were shown already in 1942 and clearly demonstrated in 1963, the already widely used term "Rh" was kept for the clinically described human antibodies which are different from the ones related to the rhesus monkey. This real factor found in rhesus macaque was classified in the Landsteiner–Wiener antigen system in honor of the discoverers.
It was recognized that the Rh factor was just one in a system of various antigens. Based on different models of genetic inheritance, two different terminologies were developed; both of them are still in use.
The clinical significance of this highly immunizing D antigen was soon realized. Some keystones were to recognize its importance for blood transfusion, hemolytic disease of the newborn, and very importantly the prevention of it by screening and prophylaxis.
The discovery of fetal cell-free DNA in maternal circulation by Holzgrieve et al. led to the noninvasive genotyping of fetal Rh genes in many countries.
Nomenclature
The Rh blood group system has two sets of nomenclatures: one developed by Ronald Fisher and R. R. Race, the other by. Both systems reflected alternative theories of inheritance. The Fisher–Race system, which is more commonly in use today, uses the CDE nomenclature. This system was based on the theory that a separate gene controls the product of each corresponding antigen. However, the d gene was hypothetical, not actual.The Wiener system used the Rh–Hr nomenclature. This system was based on the theory that there was one gene at a single locus on each of the 2 copies of chromosome 1, each contributing to production of multiple antigens. In this theory, a gene R1 is supposed to give rise to the “blood factors” Rh0, rh′, and rh″ and the gene r to produce hr′ and hr″.
Notations of the two theories are used interchangeably in blood banking. Wiener's notation is more complex and cumbersome for routine use. Because it is simpler to explain, the Fisher–Race theory has become more widely used.
DNA testing has shown that both are partially correct. There are in fact two linked genes, the RHD gene which produces a single immune specificity and the RHCE gene with multiple specificities. Thus, Wiener's postulate that a gene could have multiple specificities has been proved to be correct. On the other hand, Wiener's theory that there is only one gene has proved to be incorrect, as has the Fischer–Race theory that there are three genes, rather than the 2. The CDE notation used in the Fisher–Race nomenclature is sometimes rearranged to DCE to more accurately represent the co-location of the C and E encoding on the RhCE gene, and to make interpretation easier.
Antigens
The proteins which carry the Rh antigens are transmembrane proteins, whose structure suggest that they are ion channels. The main antigens are D, C, E, c and e, which are encoded by two adjacent gene loci, the RHD gene which encodes the RhD protein with the D antigen and the RHCE gene which encodes the RhCE protein with the C, E, c and e antigens. There is no d antigen. Lowercase "d" indicates the absence of the D antigen.Rh phenotypes are readily identified through the presence or absence of the Rh surface antigens. As can be seen in the table below, most of the Rh phenotypes can be produced by several different Rh genotypes. The exact genotype of any individual can only be identified by DNA analysis. Regarding patient treatment, only the phenotype is usually of any clinical significance to ensure a patient is not exposed to an antigen they are likely to develop antibodies against. A probable genotype may be speculated on, based on the statistical distributions of genotypes in the patient's place of origin.
R0 is today most common in Africa. The allele was thus often assumed in early blood group analyses to have been typical of populations on the continent; particularly in areas below the Sahara. Ottensooser et al. suggested that high R0 frequencies were likely characteristic of the ancient Judea Jews, who had emigrated from Egypt prior to their dispersal throughout the Mediterranean Basin and Europe on the basis of high R0 percentages among Sephardi and Ashkenazi Jews compared to native European populations and the relative genetic isolation of Ashkenazim. However, more recent studies have found R0 frequencies as low as 24.3% among some Afroasiatic-speaking groups in the Horn of Africa, as well as higher R0 frequencies among certain other Afroasiatic speakers in North Africa and among some Palestinians in the Levant.
† Figures taken from a study performed in 1948 on a sample of 2000 people in the United Kingdom.
| Rh Phenotype | CDE | Patients | Donors |
| R1r | CcDe | 37.4 | 33.0 |
| R1R2 | CcDEe | 35.7 | 30.5 |
| R1R1 | CDe | 5.7 | 21.8 |
| rr | ce | 10.3 | 11.6 |
| R2r | cDEe | 6.6 | 10.4 |
| R0R0 | cDe | 2.8 | 2.7 |
| R2R2 | cDE | 2.8 | 2.4 |
| rr″ | cEe | – | 0.98 |
| RZRZ | CDE | – | 0.03 |
| rr′ | Cce | 0.8 | – |
Hemolytic disease of the newborn
The hemolytic condition occurs when there is an incompatibility between the blood types of the mother and fetus. There is also potential incompatibility if the mother is Rh negative and the father is positive. When any incompatibility is detected, the mother often receives an injection at 28 weeks gestation and at birth to avoid the development of antibodies towards the fetus. These terms do not indicate which specific antigen-antibody incompatibility is implicated. The disorder in the fetus due to Rh D incompatibility is known as erythroblastosis fetalis.When the condition is caused by the Rh D antigen-antibody incompatibility, it is called Rh D Hemolytic disease of the newborn or Rh disease. Here, sensitization to Rh D antigens may lead to the production of maternal IgG anti-D antibodies which can pass through the placenta. This is of particular importance to D negative females at or below childbearing age, because any subsequent pregnancy may be affected by the Rh D hemolytic disease of the newborn if the baby is D positive. The vast majority of Rh disease is preventable in modern antenatal care by injections of IgG anti-D antibodies. The incidence of Rh disease is mathematically related to the frequency of D negative individuals in a population, so Rh disease is rare in old-stock populations of Africa and the eastern half of Asia, and the Indigenous peoples of Oceania and the Americas, but more common in other genetic groups, most especially Western Europeans, but also other West Eurasians, and to a lesser degree, native Siberians, as well as those of mixed-race with a significant or dominant descent from those.
- Symptoms and signs in the fetus:
- * Enlarged liver, spleen, or heart and fluid buildup in the fetus' abdomen seen via ultrasound.
- Symptoms and signs in the newborn:
- * Anemia that creates the newborn's pallor.
- * Jaundice or yellow discoloration of the newborn's skin, sclera or mucous membrane. This may be evident right after birth or after 24–48 hours after birth. This is caused by bilirubin.
- * Enlargement of the newborn's liver and spleen.
- * The newborn may have severe edema of the entire body.
- * Dyspnea or difficulty breathing.
Population data
| Population | Rh Neg | Rh Pos | Rh Neg alleles |
| African Americans | ∼ 7% | 93% | ∼ 26% |
| Albania | 10.86% | 89% | weak D 1.4% |
| Basques | 21%–36% | 65% | ∼ 60% |
| Britain | 17% | 83% | |
| China | < 1% | > 99% | |
| Ethiopians | 1%–21% | 99%–79% | |
| Europeans | 16% | 84% | 40% |
| India | 0.6%–8.4% | 99.4%–91.6% | |
| Indonesia | < 1% | > 99% | |
| Japan | < 1% | > 99% | |
| Koreans | < 1% | > 99% | |
| Madagascar | 1% | 99% | |
| Moroccans | 9.5% | 90.5% | |
| Moroccans | ∼ 29% | 71% | |
| Native Americans | ∼ 1% | 99% | ∼ 10% |
| Nigeria | 6% | 94% | |
| Saudi Arabia | 8.8% | 91.2% | 29.5% |
| Subequatorial Africa | 1%–3% | 99%–97% | |
| United States | 15% | 85% |
Inheritance
If both of a child's parents are Rh negative, the child will definitely be Rh negative. Otherwise the child may be Rh positive or Rh negative, depending on the parents' specific genotypes.The D antigen is inherited as one gene with various alleles. Though very much simplified, one can think of alleles that are positive or negative for the D antigen. The gene codes for the RhD protein on the red cell membrane. D− individuals who lack a functional RHD gene do not produce the D antigen, and may be immunized by D+ blood.
The epitopes for the next 4 most common Rh antigens, C, c, E and e are expressed on the highly similar RhCE protein that is genetically encoded in the RHCE gene, also found on chromosome 1. It has been shown that the RHD gene arose by duplication of the RHCE gene during primate evolution. Mice have just one RH gene.
The RHAG gene, which is responsible for encoding Rh-associated glycoprotein, is found on chromosome 6a.
The polypeptides produced from the RHD and RHCE genes form a complex on the red blood cell membrane with the Rh-associated glycoprotein.
Function
On the basis of structural homology it has been proposed that the product of RHD gene, the RhD protein, is a membrane transport protein of uncertain specificity and unknown physiological role. The three-dimensional structure of the related RHCG protein and biochemical analysis of the RhD protein complex indicates that the RhD protein is one of three subunits of an ammonia transporter. Three recent studies have reported a protective effect of the RhD-positive phenotype, especially RhD heterozygosity, against the negative effect of latent toxoplasmosis on psychomotor performance in infected subjects. RhD-negative compared to RhD-positive subjects without anamnestic titres of anti-Toxoplasma antibodies have shorter reaction times in tests of simple reaction times. And conversely, RhD-negative subjects with anamnestic titres exhibited much longer reaction times than their RhD-positive counterparts. The published data suggested that only the protection of RhD-positive heterozygotes was long term in nature; the protection of RhD-positive homozygotes decreased with duration of the infection while the performance of RhD-negative homozygotes decreased immediately after the infection. The overall change in reaction times was always larger in the RhD-negative group than in the RhD-positive.Origin of RHD polymorphism
For a long time, the origin of RHD polymorphism was an evolutionary enigma. Before the advent of modern medicine, the carriers of the rarer allele were at a disadvantage as some of their children were at a higher risk of fetal or newborn death or health impairment from hemolytic disease. It was suggested that higher tolerance of RhD-positive heterozygotes against Toxoplasma-induced impairment of reaction time and Toxoplasma-induced increase of risk of traffic accident could counterbalance the disadvantage of the rarer allele and could be responsible both for the initial spread of the RhD allele among the RhD-negative population and for a stable RhD polymorphism in most human populations. It was also suggested that differences in the prevalence of Toxoplasma infection between geographical regions could also explain the striking variation in the frequency of RhD-negative alleles between populations. According to some parasitologists, it is possible that the better psychomotor performance of RhD-negative subjects in the Toxoplasma-free population could be the reason for spreading of the “d allele” in the European population. In contrast to the situation in Africa and certain regions of Asia, the abundance of wild cats in Europe was very low before the advent of the domestic cat.Weak D
In serologic testing, D positive blood is easily identified. Units that are D negative are often retested to rule out a weaker reaction. This was previously referred to as Du, which has been replaced. By definition, weak D phenotype is characterized by negative reaction with anti-D reagent at immediate spin, negative reaction after 37 °C incubation, and positive reaction at anti-human globulin phase. Weak D phenotype can occur in several ways. In some cases, this phenotype occurs because of an altered surface protein that is more common in people of European descent. An inheritable form also occurs, as a result of a weakened form of the R0 gene. Weak D may also occur as "C in trans", whereby a C gene is present on the opposite chromosome to a D gene. The testing is difficult, since using different anti-D reagents, especially the older polyclonal reagents, may give different results.The practical implication of this is that people with this sub-phenotype will have a product labeled as "D positive" when donating blood. When receiving blood, they are sometimes typed as a "D negative", though this is the subject of some debate. Most "Weak D" patients can receive "D positive" blood without complications. However, it is important to correctly identify the ones that have to be considered D+ or D−. This is important, since most blood banks have a limited supply of "D negative" blood and the correct transfusion is clinically relevant. In this respect, genotyping of blood groups has much simplified this detection of the various variants in the Rh blood group system.
Partial D
It is important to differentiate weak D from partial D. Simply put, the weak D phenotype is due to a reduced number of D antigens on a red blood cell. In contrast, the partial D phenotype is due to an alteration in D-epitopes. Thus, in partial D, the number of D antigens is not reduced but the protein structure is altered. These individuals, if alloimmunized to D, can produce an anti-D antibody. Therefore, partial D patients who are donating blood should be labeled as D-positive but, if receiving blood, they should be labeled as D-negative and receive D-negative units.In the past, partial D was called 'D mosaic' or 'D variant.' Different partial D phenotypes are defined by different D epitopes on the outer surface of the red blood cell membrane. More than 30 different partial D phenotypes have been described.
Rhnull phenotype
Rhnull individuals have no Rh antigens on their red blood cells. This rare condition has been called "Golden Blood". As a consequence of Rh antigen absence, Rhnull red blood cells also lack LW and Fy5 and show weak expression of S, s, and U antigens. Red blood cells lacking Rh/RhAG proteins have structural abnormalities and cell membrane defects that can result in hemolytic anemia.Only 43 individuals have been reported to have it worldwide. The last reported carrier is a Pakistani girl named Ranam Rao. Only 9 active donors have been reported. Its properties make it attractive in numerous medical applications, but scarcity makes it expensive to transport and acquire.
Other Rh group antigens
Currently, 50 antigens have been described in the Rh group system; among those described here, the D, C, c, E and e antigens are the most important. The others are much less frequently encountered or are rarely clinically significant. Each is given a number, though the highest assigned number is not an accurate reflection of the antigens encountered since many have been combined, reassigned to other groups, or otherwise removed.Rh antibodies
Rh antibodies are IgG antibodies which are acquired through exposure to Rh-positive blood. The D antigen is the most immunogenic of all the non-ABO antigens. Approximately 80% of individuals who are D-negative and exposed to a single D-positive unit will produce an anti-D antibody. The percentage of alloimmunization is significantly reduced in patients who are actively exsanguinating.All Rh antibodies except D display dosage.
If anti-E is detected, the presence of anti-c should be strongly suspected. It is therefore common to select c-negative and E-negative blood for transfusion patients who have an anti-E. Anti-c is a common cause of delayed hemolytic transfusion reactions.