Blood type refers to the classification of blood based on the presence or absence of specific antigens—proteins or carbohydrates—on the surface of red blood cells (RBCs).[1] The two most clinically significant systems are the ABO blood group system and the Rh blood group system, which together categorize human blood into eight primary types: A+, A-, B+, B-, AB+, AB-, O+, and O-.[2][3] These classifications are essential for safe blood transfusions, organ transplants, and managing pregnancy complications, as mismatched blood can trigger severe immune reactions.[4] Overall, there are 48 recognized blood group systems comprising 398 antigens, but ABO and Rh account for the majority of transfusion-related issues.[5]The ABO system, discovered in 1901 by Karl Landsteiner, is determined by the ABO gene on chromosome 9, which has three main alleles: A, B, and O.[4] Individuals with A alleles produce A antigens (N-acetylgalactosamine added to H precursor substance), B alleles produce B antigens (galactose added), both yield AB antigens, and O alleles result in no addition, leaving only the H antigen.[2] Naturally occurring antibodies in plasma—anti-B in type A, anti-A in type B, both in type O, and none in type AB—target the missing antigens, enabling agglutination tests for typing.[4] Phenotype frequencies vary globally; for example, type O is most common (around 44% in Caucasians), while AB is rarest (about 4%).[2] Subgroups like A1 and A2 exist, with A1 expressing more antigens.[2]The Rh system, named after the Rhesus monkey in which it was first identified in 1940, involves over 50 antigens encoded by the RHD and RHCE genes on chromosome 1, with the D antigen being the most immunogenic.[3] Rh-positive individuals (about 85% of populations) express the D antigen, while Rh-negative lack it due to RHD gene deletion or inactivation.[1] Unlike ABO, Rh antibodies (primarily IgG) are not naturally occurring but develop after exposure via transfusion or pregnancy, leading to hemolytic reactions.[3] Key antigens include C/c and E/e pairs, but D typing is routine for compatibility.[3]Clinically, blood types influence transfusion practices: O- is the universal donor for RBCs due to lacking A, B, and D antigens, while AB+ is the universal recipient.[4] In obstetrics, Rh incompatibility can cause hemolytic disease of the newborn (HDN) in Rh- mothers carrying Rh+ fetuses, though prophylactic anti-D immunoglobulin has drastically reduced incidence.[3] ABO incompatibility may cause milder HDN or acute transfusion reactions, which are often fatal if untreated.[4] Emerging research also links blood types to disease susceptibilities, such as higher gastric cancer risk in type A and cardiovascular issues in non-O types, though these associations require further study.[2]
Blood Group Systems
ABO Blood Group System
The ABO blood group system is the most clinically significant blood grouping mechanism, characterized by the presence or absence of A and B antigens on the surface of red blood cells (RBCs). These antigens are complex carbohydrate structures attached to proteins or lipids on the RBC membrane, derived from a precursor known as the H antigen. The H antigen serves as the foundational structure, consisting of a chain of sugars ending in fucose attached via an α1,2 linkage to a galactose residue. In individuals with blood type O, the H antigen remains unmodified and is prominently expressed on RBCs.[2]The A and B antigens are formed by the addition of specific terminal sugar residues to the H antigen precursor through the action of glycosyltransferase enzymes encoded by alleles at the ABO locus on chromosome 9. The A allele encodes an α1,3-N-acetylgalactosaminyltransferase enzyme that transfers N-acetylgalactosamine from UDP-N-acetylgalactosamine to the terminal galactose of the H antigen, creating the A antigen. Similarly, the B allele encodes an α1,3-galactosyltransferase that adds galactose from UDP-galactose to the same position, forming the B antigen. The O allele results in a nonfunctional enzyme due to a frameshift mutation, preventing A or B antigen synthesis and leaving the H antigen intact. These enzymatic differences determine the four primary ABO phenotypes: type A (presence of A antigen), type B (presence of B antigen), type AB (presence of both A and B antigens), and type O (absence of A and B antigens).[2][6]Naturally occurring antibodies in plasma provide immune surveillance against mismatched ABO antigens, developing early in life due to exposure to similar carbohydrate structures in the environment. Individuals with blood type B produce anti-A antibodies (primarily IgM and some IgG), those with type A produce anti-B antibodies, and type O individuals produce both anti-A and anti-B antibodies, which can cause hemagglutination and hemolysis upon incompatible transfusion. Type AB individuals lack these anti-A and anti-B antibodies. Anti-H antibodies are generally absent in most people, as the H antigen is ubiquitous, but rare exceptions occur. The Bombay phenotype, resulting from a homozygous recessive hh genotype at the separate FUT1 locus on chromosome 19, prevents H antigen formation by inactivating the fucosyltransferase enzyme, thus blocking A and B antigen expression despite IA or IB alleles; affected individuals appear as type O but produce anti-H antibodies in addition to anti-A and anti-B.[2][7]Globally, the ABO phenotypes exhibit varying frequencies across populations, with type O being the most prevalent at approximately 46%, followed by type A (around 40%), type B (about 11%), and type AB as the rarest at roughly 4%; these distributions reflect evolutionary pressures and genetic drift in different ethnic groups.[2][8]
Rh Blood Group System
The Rh blood group system is one of the most clinically significant blood group systems, characterized by a complex array of antigens expressed on the surface of red blood cells. The primary antigens are carried by two homologous transmembrane proteins, RhD and RhCE, which are encoded by the closely linked RHD and RHCE genes on chromosome 1. These proteins span the red blood cell membrane multiple times and form a complex with Rh-associated glycoprotein (RhAG), contributing to the structural integrity of the membrane and the transport of ammonium and carbon dioxide. The RhCE protein expresses the antigens C, c, E, and e through allelic variations, while the RhD protein specifically carries the D antigen. Over 50 distinct Rh antigens have been identified, but the D antigen (Rh1) is the most immunogenic, capable of eliciting strong immune responses in individuals lacking it.[9][10][11]The presence or absence of the RhD antigen determines an individual's Rh status, classifying them as Rh-positive or Rh-negative, respectively. Approximately 85% of Caucasians are Rh-positive due to the expression of functional RhD protein, whereas the remaining 15% lack this antigen because of a deletion or silencing of the RHD gene. Rh-negative individuals can produce anti-D antibodies upon exposure to RhD-positive blood, such as through transfusion or pregnancy, leading to potential hemolytic reactions. In combination with the ABO system, this results in eight main blood types, such as O+ and A-. The RhD antigen's high immunogenicity makes it the primary focus of Rh typing in clinical practice.[3][10]Variants of the RhD antigen, including weak D and partial D, arise from mutations in the RHD gene that alter antigen expression or structure. Weak D variants involve reduced quantitative expression of the RhD protein, often due to missense mutations, resulting in weaker reactions with anti-D reagents in serological testing but generally not eliciting anti-D formation in recipients. Partial D variants, in contrast, feature qualitative changes, such as amino acid substitutions or hybrid alleles with RHCE, leading to altered epitopes that can react variably with different anti-D antibodies and potentially immunize Rh-negative individuals if the missing epitopes are present on donor cells. These variants require molecular genotyping for accurate identification to guide safe transfusion practices.[12][13]The Rh null phenotype represents an extremely rare condition where all Rh antigens are absent due to mutations in the RHAG gene (regulator type) or homozygous inheritance of silent RHD and RHCE alleles (amorph type), affecting fewer than 50 individuals worldwide. This absence disrupts the red blood cell membrane structure, leading to chronic hemolytic anemia characterized by spherocytosis, increased osmotic fragility, and shortened red cell survival. Affected individuals often require supportive transfusions, though their lack of Rh antigens makes them universal donors for Rh typing but increases risks of alloimmunization to other antigens. Rh incompatibility, particularly involving the D antigen, can also contribute to hemolytic disease of the newborn if an Rh-negative mother is sensitized by an Rh-positive fetus.[14][15][16]
Other Blood Group Systems
Beyond the ABO and Rh systems, the International Society of Blood Transfusion (ISBT) recognizes 48 blood group systems comprising 367 red cell antigens as of August 2025.[17] These systems are defined by genetically determined antigens on red blood cell surfaces, with clinical relevance primarily in transfusion medicine due to their potential to elicit alloantibodies causing hemolytic reactions.[18] While ABO and Rh dominate routine compatibility testing, extended phenotyping panels incorporate antigens from these other systems to mitigate risks in sensitized patients.[19]The Kell blood group system (ISBT 006) includes 38 antigens carried on a type II transmembrane glycoprotein, with K (KEL1) and k (KEL2) being the most prevalent and immunogenic after those in ABO and Rh.[19] The K antigen is highly immunogenic, capable of provoking strong IgG alloantibodies that cause severe hemolytic transfusion reactions and hemolytic disease of the fetus and newborn (HDFN).[20] The rare Kell-null (K0) phenotype, lacking all Kell antigens due to mutations in the KEL gene, is associated with reduced Kell protein expression but no overt hematologic abnormalities in most cases.[19]The Duffy blood group system (ISBT 008) features FY^A and FY^B antigens on a glycoprotein that also functions as a chemokine receptor (DARC).[21] The Fy(a-b-) phenotype, resulting from a FY*B^ES allele silencing expression in erythrocytes, predominates in populations of African ancestry (up to 70%) and confers resistance to Plasmodium vivaxmalaria invasion by eliminating the parasite's receptor.[21] Duffy antibodies, typically IgG, can cause delayed hemolytic transfusion reactions and mild HDFN, though their clinical impact is often less severe than Kell.[22]In the Kidd blood group system (ISBT 009), JK^A and JK^B antigens are expressed on a urea transporter protein (UT-B), with anti-JK^A and anti-JK^B antibodies notorious for causing delayed hemolytic transfusion reactions due to their ability to evade detection by standard methods and re-emerge post-transfusion.[23] These IgG antibodies, often complement-activating, lead to extravascular hemolysis days to weeks after incompatible transfusion, emphasizing the need for Kidd-compatible units in antibody-positive patients.[24]The MNS blood group system (ISBT 002), one of the most complex with over 50 antigens, includes high-prevalence M (MNS1), N (MNS2), S (MNS4), and s (MNS5) determinants on glycophorins A and B.[25] Anti-M and anti-N are usually IgM and clinically benign, but anti-S and anti-s (IgG) pose risks of acute or delayed hemolytic reactions and HDFN, particularly in multiparous women.[25]The Lutheran blood group system (ISBT 005) encompasses 28 antigens, primarily Lu^A (LU1) and Lu^B (LU2) on the basal laminaglycoprotein (LU), with Lu^B expressed in nearly all individuals.[26] Lutheran antibodies are uncommon and typically cause mild delayed hemolytic transfusion reactions, though rare cases of severe HDFN have been reported; the dominant inhibitor (In(Lu)) phenotype suppresses expression without clinical sequelae.[26]Among rarer systems, the Diego blood group (ISBT 010) includes DI^A and DI^B antigens on the anion exchanger band 3 (AE1), with DI^A prevalent in East Asians and Native Americans (up to 50% in some groups).[27] Anti-DI^A can induce HDFN or hemolytic transfusion reactions, as seen in severe neonatal cases requiring exchange transfusion.[28] The P1PK system (ISBT 003), involving glycosphingolipid antigens P1, P^k, and NOR1, features the rare p phenotype (lacking all three) associated with increased norovirus susceptibility but minimal transfusion risks beyond anti-PP1P^k antibodies causing occasional intravascular hemolysis.[29]
Genetics and Inheritance
Genetic Basis of ABO and Rh Systems
The ABO blood group system is determined by a single genetic locus on the long arm of chromosome 9 at position 9q34.1-q34.2.[2] This locus encodes a glycosyltransferaseenzyme responsible for adding specific sugar residues to the H antigen precursor on red blood cell surfaces. The three principal alleles—A, B, and O—arise from variations in this gene, with the A and B alleles differing by seven nucleotide substitutions, four of which result in amino acid changes that alter enzyme specificity.[2] The A allele encodes an enzyme that transfers N-acetylgalactosamine, while the B allele transfers galactose; both A and B alleles exhibit codominance, leading to the AB phenotype when inherited together.[2] In contrast, the O allele is characterized by a frameshift mutation—a single guanine deletion at nucleotide position 261 in exon 6 (tagged by the single nucleotide polymorphism rs8176719)—which introduces a premature stop codon and produces a nonfunctional enzyme, rendering the O allele recessive to A and B.[2][30]Expression of ABO antigens requires the H antigen precursor, synthesized by the fucosyltransferase encoded by the FUT1 gene on chromosome 19 at 19q13.3.[31] This gene catalyzes the addition of fucose to a precursor glycolipid or glycoprotein, forming the H structure essential for subsequent A or B antigen modification.[31] In cases of homozygous FUT1 mutations (h/h genotype), as seen in the rare Bombay phenotype, no H antigen is produced, resulting in an epistatic interaction that prevents ABO antigen expression regardless of the ABO genotype, phenotypically mimicking blood type O.[31]The Rh blood group system is governed by the RH locus on the short arm of chromosome 1 at 1p36.11, comprising two highly homologous genes, RHD and RHCE, arranged in tandem and separated by approximately 35 kb.[3] The RHD gene encodes the RhD protein, responsible for the D antigen, while RHCE encodes the RhCE protein, which carries the C/c and E/e antigens through alternative splicing and polymorphisms.[3] The RhD-negative phenotype in most populations, particularly Europeans, results from a complete deletion of the RHD gene, occurring via unequal homologous recombination between flanking "Rhesus boxes"—noncoding sequences that bookend RHD.[3] Variants such as the rare D-- phenotype arise from hybrid RHD-RHCE genes, where recombination replaces portions of RHCE exons with RHD sequences, altering antigen expression and potentially leading to partial D or weakened epitopes.[3]Allele frequencies for both ABO and Rh systems vary significantly by ancestry, reflecting evolutionary pressures and genetic drift. In the ABO system, the frequency of the B antigen is notably higher in Asian populations (27%) compared to Europeans (9%) or Africans (20%), contributing to elevated B and AB phenotypes in East and South Asian groups.[2] For the Rh system, the RHD deletion allele (associated with Rh-negative) reaches frequencies of 0.35–0.40 in Europeans but is rare (less than 0.05) in Asians and Africans, where alternative mutations like pseudogenes or hybrids predominate.[3]
Inheritance Patterns and Phenotypes
The inheritance of blood types follows Mendelian principles, with the ABO system governed by a single gene on chromosome 9 featuring three alleles: I^A (producing A antigen), I^B (producing B antigen), and i (producing no antigen, resulting in type O).[2] The I^A and I^B alleles exhibit codominance, meaning both antigens are expressed if present together, while the i allele is recessive to both.[2] This leads to four main phenotypes: type A (genotypes I^A I^A or I^A i), type B (I^B I^B or I^B i), type AB (I^A I^B), and type O (i i).[2]To illustrate, consider parents with genotypes I^A i (type A) and I^B i (type B). A Punnett square predicts the offspring phenotypes as follows:
I^B
i
I^A
I^A I^B (AB)
I^A i (A)
i
I^B i (B)
i i (O)
Each outcome occurs with 25% probability, demonstrating the equal segregation of alleles.[2]The Rh system is determined by the presence or absence of a functional RHD gene on chromosome 1. Individuals with at least one functional copy of RHD are RhD-positive, while those lacking functional RHD (e.g., homozygous for the gene deletion) are RhD-negative.[3] This is traditionally simplified as a dominant D over recessive d model, but molecularly reflects the inheritance of functional versus non-functional RHD alleles.[32]Inheritance involves haplotypes combining RhD status with other Rh antigens (C/c, E/e), such as R^1r (DCe/dce), where cis effects occur when antigens like D and Ce are on the same chromosome, influencing antigen density and antibody reactivity compared to trans configurations across chromosomes.[10][32]Rare non-Mendelian cases include chimerism, arising from fusion of two zygotes (e.g., dizygotic twins sharing circulation), and mosaicism, from post-zygotic mutations, both leading to mixed cell populations and ABO phenotype-genotype discrepancies detectable via short tandem repeat analysis.[33] For instance, a chimeric individual may show AB red blood cells but a B/O genotype due to dual paternal contributions.[33]Phenotype-genotype discrepancies also occur with variant alleles like cis-AB, a rare ABO allele encoding a single glycosyltransferase that inefficiently produces both A and B antigens, often resulting in a weak AB phenotype (e.g., A2B3) that mimics mixed inheritance patterns.[34] This can cause apparent paradoxes, such as an O child from an AB parent, resolvable through enzymatic kinetic studies showing reduced antigen expression.[34]
Global Distribution and Prevalence
Worldwide Prevalence of ABO and Rh Types
The worldwide prevalence of ABO and Rh blood types exhibits significant variation, influenced by genetic drift, migration, and historical population movements, with no single uniform global distribution but approximate averages derived from large-scale blood donor surveys and population studies. Based on compilations from international blood services, the most common type is O positive, accounting for approximately 42% of the global population, followed by A positive at 31%, B positive at 15%, AB positive at 5%, O negative at 3%, A negative at 2.5%, B negative at 1%, and AB negative at 0.5%.[35] These figures reflect data aggregated from diverse regions up to 2023, including surveys by organizations like the American Red Cross and World Population Review, though exact percentages can shift slightly by study methodology.[36] Overall, type O dominates globally at around 45% across ABO groups alone, underscoring its ancestral prevalence in early human populations.[37]Regional patterns highlight stark contrasts: in indigenous populations of the Americas, type O reaches up to 90-100%, a near-monomorphic trait likely stemming from founder effects during ancient migrations across Beringia.[38] In contrast, type A predominates in Europe, comprising 35-40% of the population, particularly in northern regions where it may have conferred selective advantages against certain pathogens.[36] For the Rh factor, negative status is rare globally at about 7%, but it peaks among the Basques at 35-50%, possibly due to genetic isolation in prehistoric Iberian refugia, while it is nearly absent in Asian populations at less than 1%.[39][40]Evolutionary pressures have shaped these distributions, with type O associated with lower thrombosis risk compared to A and B types, which carry higher von Willebrand factor levels and thus elevated clotting tendencies.[41] Conversely, A and B antigens may offer protection against specific infections, while blood type O may confer higher susceptibility to others such as norovirus, contributing to balanced polymorphism over millennia.[42] Global migration may lead to shifts in local blood type distributions in multicultural urban centers.
Blood Type
Approximate Global Prevalence (%)
O+
42
A+
31
B+
15
AB+
5
O-
3
A-
2.5
B-
1
AB-
0.5
Regional and Ethnic Variations
Blood type distributions exhibit significant regional and ethnic variations, shaped by genetic drift, founder effects, and historical migrations. In India, the ABO frequencies show a predominance of group B, reflecting South Asian genetic patterns, with overall nationwide data indicating O at 37.12%, B at 32.26%, A at 22.88%, and AB at 7.74%; Rh-positive individuals comprise 94.61% of the population.[43] These proportions vary regionally, with B more common in the north and O prevailing elsewhere, influenced by ancient population movements. In Japan, group A is the most frequent at approximately 40%, followed by O at 30%, B at 20%, and AB at 10%, while Rh-negative is rare at less than 0.5%.[44] Nigeria demonstrates a strong prevalence of group O at 52.93%, with A at 22.77%, B at 20.64%, AB at 3.66%, and Rh-positive at 94.90%, consistent with West African profiles where O offers potential selective advantages against certain infections.[45]Ethnic groups further highlight these differences. Among Ashkenazi Jews, group A frequencies are elevated compared to some non-European populations, aligning closer to Northern European patterns at around 40%, though exact distributions vary by subgroup due to historical admixture.[46] Native American populations show near-exclusive dominance of group O, with frequencies exceeding 90% in many indigenous groups, and virtually all being Rh-positive, a pattern attributed to founder effects from ancient Beringian migrations.[47]Historical migrations have profoundly influenced these variations. The B allele's spread across Central Asia and into South Asia is linked to ancient trade routes like the Silk Road, where it appears at higher frequencies in Turkic and related populations, facilitating gene flow from East to West.[48] Similarly, the Rh-negative allele, prevalent in Europeans at 15-40%, likely arose from genetic bottlenecks during prehistoric population expansions and isolations, such as in the Basque region, where drift amplified its frequency to 47.2%.[39]Recent surveys from the 2020s underscore how urbanization affects donor pools, with urban areas showing higher donation rates but potential biases in blood type representation due to demographic shifts toward younger, more mobile populations, potentially reducing diversity in rural-sourced types like higher O in indigenous groups.[49]
Blood transfusion compatibility is essential to prevent immune-mediated reactions that can lead to hemolysis, organ damage, or death. The primary systems governing compatibility are ABO and Rh, where red blood cells (RBCs) from the donor must not express antigens against which the recipient has pre-existing antibodies. In the ABO system, individuals produce antibodies against the A and/or B antigens they lack: type A has anti-B antibodies, type B has anti-A, type AB has neither, and type O has both. Thus, donor RBCs must lack the antigens targeted by the recipient's antibodies to avoid agglutination and complement activation.[50]The compatibility rules for ABO types are as follows:
Type O individuals serve as universal donors for RBC transfusions because their cells lack A and B antigens, minimizing reaction risk across recipients. Conversely, type AB recipients can accept RBCs from any ABO type due to the absence of anti-A and anti-B antibodies. These rules ensure that transfused RBCs do not trigger an immune response in the recipient.[51]For the Rh system, the key antigen is RhD. Rh-negative (Rh-) individuals may develop anti-D antibodies if exposed to Rh-positive (Rh+) blood, leading to sensitization. Therefore, Rh- recipients should receive only Rh- blood to prevent antibody formation and future reactions, while Rh+ recipients can safely receive either Rh+ or Rh- blood, as they lack anti-D antibodies. This matching reduces the risk of hemolytic complications, particularly in females of childbearing age.[50]Beyond ABO and Rh typing, cross-matching confirms compatibility by testing for unexpected antibodies. The major cross-match mixes the recipient's serum with donor RBCs to detect agglutination from recipient antibodies against donor antigens. The minor cross-match, less commonly performed in modern practice due to low plasma volumes in RBC components, tests donor serum against recipient RBCs for donor antibodies. A compatible cross-match indicates no detectable reaction, allowing safe transfusion.[52]Transfusion reactions related to incompatibility vary in timing and severity. Acute hemolytic reactions, typically from ABO mismatches, occur within 24 hours and involve rapid intravascular hemolysis, fever, chills, back pain, and potential renal failure due to complement activation and cytokine release. Delayed hemolytic reactions, often involving Rh or other antigens like Kidd, manifest 3–14 days post-transfusion with extravascular hemolysis, jaundice, and milder symptoms, resulting from anamnestic antibody responses. These reactions underscore the need for precise matching.[53]In emergencies, such as trauma or massive hemorrhage where typing is unavailable, type O-negative blood is used as the universal donor for RBCs, as it lacks A, B, and RhD antigens. For whole blood transfusions in austere settings like battlefield or prehospital care, low-titer group O whole blood (LTOWB)—with anti-A and anti-B antibody titers below 1:256—is preferred to further reduce reaction risks while providing plasma, RBCs, and platelets in one unit. These protocols prioritize rapid volume resuscitation while awaiting type-specific blood.[54]
Hemolytic Disease of the Newborn
Hemolytic disease of the newborn (HDN), also known as erythroblastosis fetalis, is an immune-mediated condition arising from blood group incompatibility between a pregnant individual and the fetus, leading to the destruction of fetal red blood cells (RBCs). The primary cause is Rh incompatibility, where an Rh-negative (Rh-) mother develops antibodies against the Rh(D) antigen on the fetal RBCs if the fetus is Rh-positive (Rh+). These maternal IgG anti-D antibodies cross the placenta and bind to fetal Rh+ RBCs, triggering hemolysis, anemia, hyperbilirubinemia, and potentially severe complications such as hydrops fetalis or kernicterus.[55] A milder form can occur due to ABO incompatibility, typically when an O-group mother has an A or B fetus, but it rarely leads to significant morbidity.[56]Risk factors for Rh HDN include maternal Rh- status, paternal or fetal Rh+ status, and prior sensitizing events such as a previous Rh+ pregnancy, miscarriage, abortion, or ectopic pregnancy that expose the mother to fetal blood. Sensitization occurs in about 15% of Rh- mothers after an Rh+ delivery without prophylaxis, increasing the risk of HDN in subsequent pregnancies. ABO HDN risk is higher in group O mothers but is generally self-limited due to lower antibody titers and the protective expression of A and B antigens on other fetal tissues.[57]Prevention of Rh HDN relies on the administration of Rho(D) immune globulin (RhoGAM), a preparation of anti-D antibodies that prevents maternal sensitization by clearing fetal RBCs from the maternal circulation before an immune response develops. Prophylaxis is recommended for all Rh- pregnant individuals at 28 weeks gestation (300 μg dose) and postpartum within 72 hours if the newborn is Rh+, reducing sensitization rates from 12-13% to 1-2%. Additional doses are given after sensitizing events like amniocentesis or bleeding. This intervention has decreased Rh HDN incidence dramatically since its introduction in the 1960s.[58][59]Diagnosis of HDN involves antenatal screening for maternal alloantibodies via indirect Coombs test and antibody titers, with fetal assessment using middle cerebral artery Doppler ultrasound to detect anemia or amniocentesis for bilirubin levels in amniotic fluid. Postnatally, it is confirmed by direct Coombs test on cord blood, elevated bilirubin, reticulocytosis, and spherocytes on peripheral smear. Severity is graded by hemoglobin levels and bilirubin trends.[55][57]Treatment depends on severity: mild cases are managed with phototherapy to reduce bilirubin and IV immunoglobulin to block hemolysis, while severe antenatal anemia requires intrauterine transfusion of Rh- blood via the umbilical vein. Postnatally, exchange transfusion may be needed for profound anemia or hyperbilirubinemia unresponsive to phototherapy, with outcomes improving survival to over 90% in treated cases.[55]Prior to widespread RhoGAM use in the 1960s, Rh HDN affected approximately 45 per 10,000 births in the United States, with high rates of fetal loss or neonatal morbidity. Post-prophylaxis, in settings with routine prophylaxis, the incidence has fallen to about 0.047% for Rh(D)-mediated cases, though ABO HDN, while more common, typically has low clinical impact in incompatible pregnancies.[60][55]
Associations with Disease Susceptibility
Blood types, particularly within the ABO system, have been associated with varying susceptibilities to several diseases, independent of transfusion contexts. These links often stem from the influence of ABO antigens on biological processes such as coagulation, pathogen binding, and immune responses. Genome-wide association studies (GWAS) and meta-analyses have provided evidence for these associations, highlighting how specific blood groups modulate disease risk through mechanisms like altered endothelial function or microbial adhesion. These associations are primarily correlative and mechanistic studies are ongoing to establish causality.In cardiovascular health, individuals with blood type O exhibit a lower risk of venous thromboembolism (VTE) compared to those with non-O types. A meta-analysis of multiple cohort studies reported a pooled adjusted odds ratio (OR) of 1.31 (95% CI: 1.02–1.68) for VTE in non-O versus O blood groups, indicating approximately a 20-30% reduced odds for type O individuals.[61] This protective effect is attributed to lower levels of von Willebrand factor and factor VIII in type O, which reduce clotting propensity.[62]Regarding infectious diseases, blood type O is linked to higher susceptibility to norovirus infections. A systematic review and meta-analysis of 14 studies involving over 5,000 participants found that type O individuals had an increased risk compared to non-O types (OR 1.45, 95% CI: 1.15–1.82), as the unmodified H antigen on O erythrocytes and mucosal cells serves as a binding site for the virus.[63] Conversely, for severe Plasmodium falciparum malaria, blood group O confers protection through reduced rosetting—a process where infected erythrocytes bind uninfected ones, exacerbating vascular obstruction. In a study of 567 Malian children, type O was associated with a 66% lower odds of severe malaria (OR 0.34, 95% CI: 0.19–0.61), with parasite isolates from O individuals showing significantly lower rosetting rates.[64] Non-O groups (A and B) facilitate stronger rosetting due to compatible antigens, increasing severe disease risk.[65]For cancers, blood group A is associated with elevated gastric cancer risk. A meta-analysis of 40 studies encompassing 33,613 cases and over 2.4 million controls reported an OR of 1.19 (95% CI: 1.13–1.25) for type A versus O, representing about a 20% increased risk, potentially due to enhanced Helicobacter pylori adhesion to A antigens.[66] In contrast, type O appears protective against pancreatic cancer. Two large prospective cohort studies involving over 100,000 participants found that non-O types had higher risks, with type A showing a 37% increase (relative risk 1.37, 95% CI: 1.14–1.66) compared to O, linked to ABO glycosyltransferase variants influencing inflammation and tumor progression.[67]During the COVID-19 pandemic, blood type O was modestly associated with lower disease severity. A meta-analysis of 22 studies covering over 1.2 million individuals reported an OR of 0.91 (95% CI: 0.85–0.99) for infection risk in type O versus non-O, with similar trends for reduced severity in hospitalized cohorts from 2020–2022 (pooled OR ≈0.85 for severe outcomes).[68] This may relate to lower viral entry facilitation by O antigens on respiratory epithelia.Recent genomic research has further elucidated these ties through GWAS identifying ABO variants as influencers of systemic inflammation. For example, a 2024 study linked the ABO rs657152 variant to differences in C-reactive protein levels associated with COVID-19 severity.[69]
Testing and Identification
Serological Blood Typing Methods
Serological blood typing, also known as phenotyping, relies on the principle of hemagglutination, where specific antibodies bind to antigens on red blood cells (RBCs), causing visible clumping or agglutination that indicates the blood type. This method determines ABO and Rh(D) antigens primarily through forward and reverse typing procedures, using monoclonal or polyclonal antisera. It remains the gold standard for routine blood grouping in transfusion medicine due to its direct assessment of antigen expression on RBC surfaces.[70]Forward typing involves mixing a suspension of the patient's RBCs with reagent antisera containing anti-A, anti-B, and anti-D antibodies. Agglutination observed with anti-A indicates the presence of A antigen (type A or AB), with anti-B for B antigen (type B or AB), and with anti-D for RhD-positive status; no agglutination with any suggests type O and RhD-negative. This direct antigen detection is performed at room temperature or 37°C, with results read macroscopically after centrifugation in most cases. For RhD typing, if initial testing shows no agglutination, a weak D test may follow using anti-human globulin (AHG) to detect low-level D antigen expression.[71][72]Reverse typing confirms forward results by testing the patient's serum or plasma for expected ABO antibodies against known A and B reagent RBCs. For instance, type A serum should agglutinate B cells but not A cells due to anti-B antibodies, while type O serum agglutinates both. This step verifies antibody presence, which typically develops by 3-6 months of age, and discrepancies between forward and reverse typing prompt further investigation for subgroups or acquired conditions. Rh typing does not include a reverse component, as anti-D antibodies are not naturally occurring.[71][70]Common techniques for serological typing include the tube method, slide test, and gel column agglutination. The tube method, a traditional manual approach, suspends RBCs in saline and adds antisera in test tubes, followed by centrifugation and resuspension to observe agglutination; it offers high sensitivity but requires 10-20 minutes and skilled interpretation. The slide method provides rapid results (5-10 minutes) by mixing drops of blood and antisera on a slide and tilting for immediate agglutination, though it is less sensitive and prone to drying artifacts, limiting its use to emergencies. Gel column agglutination, introduced in the 1980s, uses microcolumns filled with gel or dextran-acrylamide matrix to trap agglutinates during low-speed centrifugation, improving standardization and reducing subjective reading; it takes 10-45 minutes and is widely adopted for its accuracy in detecting weak reactions. Modern laboratories increasingly employ automated systems, such as those integrating gel technology with robotic pipetting and digital imaging, to process high volumes while minimizing errors.[73][70][74]Quality controls are essential to validate results and detect interferences. An auto-control, mixing patient serum with their own RBCs, identifies autoantibodies that could cause non-specific agglutination mimicking true reactions. For Rh typing, a control reagent (e.g., saline or monoclonal control) ensures no false positives from improper technique, while weak D testing incorporates AHG controls to confirm indirect antiglobulin reactions. Positive and negative controls using known typed cells are run alongside patient samples to verify reagent potency and procedural integrity.[75][76]Limitations of serological methods include reduced reliability in certain populations. In newborns, ABO antigens are expressed on RBCs but at lower density than in adults, potentially leading to weaker reactions in forward typing; reverse typing is unreliable due to absent antibodies until 3-6 months of age. Elderly individuals may exhibit weakened or absent antibody responses due to conditions like hypogammaglobulinemia, causing discrepancies in reverse typing; forward typing is generally reliable. Additionally, conditions like hematologic malignancies or massive transfusions may alter antigen density or introduce extraneous antibodies, necessitating complementary approaches for resolution.[73][70]
Molecular Genotyping Techniques
Molecular genotyping techniques utilize DNA analysis to determine blood group alleles, providing a precise alternative to serological methods, particularly in cases where antibody-based testing yields ambiguous results due to weak or variant antigen expression. These methods involve amplifying and analyzing specific genetic sequences associated with blood group systems, such as the ABO and Rh loci, to identify nucleotide variations that define antigen phenotypes. By directly examining the underlying genetic code, molecular approaches can resolve discrepancies that arise from factors like recent transfusions or chimerism, enabling accurate prediction of blood types even when red blood cell surfaces lack clear serological markers.[77]For the ABO system, polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and Sanger sequencing are established techniques to detect key allelic variants. PCR-RFLP amplifies targeted exons of the ABO gene and digests the products with restriction enzymes to distinguish alleles based on fragment patterns; for instance, it identifies the common O allele through detection of the 261delG deletion, which introduces a frameshift mutation leading to a nonfunctional glycosyltransferase. Sequencing provides higher resolution for rare or novel variants, confirming genotypes in heterozygous individuals or those with weak A or B subgroups. These methods are particularly valuable for resolving ABO discrepancies in up to 1-2% of routine typings where serology fails due to variant alleles.[78][79]Rh genotyping employs real-timePCR to detect large structural variants, such as the complete RHD gene deletion responsible for the RhD-negative phenotype in most populations, by quantifying gene copy number through hybridization probes. For detecting single nucleotide polymorphisms and hybrid alleles in RHCE, array-based platforms like BeadChips analyze multiple variants simultaneously, identifying weak D or partial D expressions that may not react reliably in serological assays. These techniques map over 200 known Rh variants, aiding in the differentiation of serological weak D types that require specific transfusion strategies.[80][81]Molecular genotyping finds key applications in prenatal RhD typing from maternal plasma to guide anti-D immunoglobulin prophylaxis, reducing unnecessary administration by up to 40% in RhD-negative pregnancies; in chronically transfused patients, such as those with sickle cell disease, to select phenotype-matched units and prevent alloimmunization; and in bone marrow donor registries to ensure compatibility beyond ABO and Rh for extended antigen matching. Advantages include the ability to predict weak or absent antigen expression from genotype alone, applicable in 10-20% of complex serological cases involving polytransfused individuals or ethnic minorities with high variant frequencies. Recent advances in the 2020s incorporate next-generation sequencing (NGS) panels that cover more than 30 blood group systems, including rare antigens like Kell and Duffy, with targeted capture of exons and flanking regions for comprehensive variant detection at costs ranging from $100 to $500 per test, enhancing scalability for large-scale donor screening. As of 2024, molecular methods have facilitated the discovery of new blood group systems, such as the MAL system, underscoring their utility in comprehensive antigen profiling across more than 30 recognized systems.[82][77][83][84]
Historical Development
Discovery of ABO System
The earliest attempts at blood transfusion date back to the 17th century, with the first recorded human transfusion occurring in 1667 when French physician Jean-Baptiste Denis transfused lamb's blood into a 15-year-old boy suffering from fever.[85] These early procedures, often involving animal-to-human or untyped human-to-human transfers, were highly dangerous due to unknown compatibility issues, resulting in frequent agglutination reactions and deaths; historical reviews indicate mortality rates approaching 50% in documented cases before systematic typing was introduced.[86]In 1900, Austrian pathologist Karl Landsteiner conducted pivotal experiments at the University of Vienna by separating red blood cells and serum from blood samples of his colleagues and mixing them to observe agglutination patterns.[87] This led to the identification of three distinct blood groups—A, B, and C (later renamed O for "ohne," meaning without in German)—based on the presence of specific isoagglutinins (antibodies) in the serum that caused clumping of incompatible red cells.[88] Landsteiner published these findings in 1901, establishing the foundational principles of blood group compatibility and revolutionizing transfusion medicine by explaining why prior attempts often failed.[87]Building on Landsteiner's work, his colleagues Alfred von Decastello and Adriano Sturli identified the fourth blood group, AB, in 1902 through similar serological testing, completing the ABO system.[87] The first documented transfusion using ABO typing and crossmatching was performed in 1907 by American physician Reuben Ottenberg at Mount Sinai Hospital in New York, marking a practical application that confirmed the system's role in preventing hemolytic reactions.[85] Early nomenclature evolved from terms like "isoagglutinins" to the standardized A, B, AB, and O designations, formalized internationally in the 1920s.[88] For his discovery, Landsteiner received the Nobel Prize in Physiology or Medicine in 1930, recognizing the ABO system's profound impact.[87]The introduction of ABO typing dramatically improved transfusion safety by enabling compatible donor-recipient matching, significantly reducing mortality rates from historical highs approaching 50% in early attempts.[87][89]
Evolution of Rh System Understanding
In 1939, Philip Levine and Rufus E. Stetson reported a severe transfusion reaction in a woman following the delivery of a stillborn fetus, attributing it to an immune response triggered by fetal-maternal blood incompatibility, which laid the groundwork for understanding Rh-related issues.[9] This observation suggested the presence of a new antigen absent in the mother but present in the fetus and donor blood. The following year, in 1940, Karl Landsteiner and Alexander S. Wiener identified the Rh factor by immunizing rabbits with blood from rhesus monkeys, producing an antibody that agglutinated red blood cells from about 85% of humans; they named it "Rh" after the rhesus monkey source, and subsequent studies confirmed it as the human anti-D antibody.[90] By 1941, Louis K. Diamond recognized hemolytic disease of the newborn (HDN) as a clinical entity linked to Rh incompatibility, building on earlier descriptions of erythroblastosis fetalis and emphasizing its immune-mediated nature.[55]In the 1940s, genetic models for the Rh system emerged amid debate. Robert R. Race and Ronald A. Fisher proposed a model of three closely linked genes (DCE) controlling Rh antigens, contrasting with Alexander S. Wiener's earlier multiple-allele hypothesis at a single locus; family studies and serological data increasingly supported the Fisher-Race model, which better explained observed inheritance patterns and antigen combinations.[10] This period marked a shift toward a more precise genetic framework for Rh variability.Molecular advances accelerated in the 1990s with the cloning of the RHD gene, responsible for the D antigen, from RhD-positive individuals, revealing it as distinct from the RHCE gene encoding C/c and E/e antigens.[91] In the 2000s, full sequencing of introns and variants in RHD and RHCE genes enabled detailed mapping of polymorphisms, improving identification of weak D and partial D variants.[92] A key clinical milestone was the 1968 development of RhoGAM (anti-D immunoglobulin), which prevents maternal Rh sensitization and has reduced HDN incidence by over 90%.[55]
Sociocultural Aspects
Blood Type in Culture and Personality Theories
In East Asian cultures, particularly Japan, South Korea, Taiwan, and China, blood type has become a popular framework for inferring personality traits, akin to astrology in Western societies. This pseudoscientific belief, known as ketsueki-gata in Japanese, posits that an individual's ABO blood group influences their temperament, behavior, and interpersonal compatibility.[93] Despite lacking empirical support, the theory permeates daily life, media, and social interactions.The concept originated in Japan in 1927 when psychologist Takeji Furukawa published "The Study of Temperament Through Blood Type," proposing correlations between blood groups and character based on limited surveys of schoolchildren.[94] Furukawa's work suggested that type A individuals are organized, cautious, and perfectionistic; type B people are creative, independent, and sometimes selfish; type AB individuals are eccentric, rational, and adaptable; while type O personalities are confident, ambitious, and resilient leaders.[94] These stereotypes gained traction through books and articles, evolving into a cultural staple by the mid-20th century. A 1986 survey of Japaneseuniversity students found that 96% believed blood types influenced personality, reflecting widespread acceptance.[95]In Japan, blood type personality theory influences social and professional spheres, including dating profiles on apps and job compatibility assessments, where type A is favored for teamwork roles and type B for innovative positions.[96] Surveys indicate that over 70% of young adults consider blood type when forming relationships or evaluating colleagues.[96]The theory spread to South Korea in the 1970s, popularized by mass media and journalist Masahiko Nomi's bestselling books, which echoed Japanese traits but emphasized entertainment applications.[97] It has also gained popularity in Taiwan and China, where it appears in entertainment, matchmaking, and personality quizzes. In Korean pop culture, celebrities often disclose their blood types on variety shows, with type O portrayed as charismatic leaders and type B as passionate free spirits, influencing fan perceptions and matchmaking discussions.[97]Globally, blood type has inspired lifestyle fads like the blood type diet, introduced by naturopath Peter D'Adamo in his 1996 book Eat Right 4 Your Type, which claims type O individuals thrive on high-protein diets mimicking hunter-gatherer ancestors, while type A benefits from vegetarianism.[98] Scientific reviews, including a 2014 University of Toronto study of over 1,400 participants, have debunked these claims, finding no evidence that blood type affects dietary responses or health outcomes.[99]These modern beliefs echo ancient ideas linking bodily fluids to temperament. In contemporary culture, blood type manifests in merchandise like personality-themed stationery and apparel in Japan, as well as horoscope-style forecasts in magazines that blend it with zodiac signs for daily advice.[95]
Ethical and Legal Considerations in Blood Typing
In medical contexts, informed consent is mandatory prior to blood typing procedures, ensuring patients understand the purpose, risks, and benefits of testing, particularly when it involves genetic implications.[100] In the United States, the Health Insurance Portability and Accountability Act (HIPAA) protects blood type information as part of protected health information when linked to individual identifiers, safeguarding genetic data from unauthorized disclosure.[101] This framework extends to broader genetic protections under the Genetic Information Nondiscrimination Act (GINA), which prevents misuse of such data in health insurance and employment decisions.[102]Concerns over discrimination based on blood type have arisen globally, particularly in employment contexts where stereotypes influence hiring. In Japan, studies document labor market biases against certain blood types, such as type B, leading to reported instances of applicants being overlooked due to perceived personality traits, prompting calls for legal safeguards.[103] Blood typing can also indirectly infer ancestry, raising risks of ethnic or racial discrimination; for example, population-specific distributions of ABO types have been misused in ancestry testing, potentially exacerbating biases in employment or social contexts.[104] Legal bans on genetic discrimination, like GINA, aim to mitigate such inferences by prohibiting employers from requesting or using genetic information, including blood types.[102]In organ donation, ABO and Rh blood type compatibility is a primary criterion for allocation under United Network for Organ Sharing (UNOS) policies, prioritizing matches to maximize transplant success and minimize rejection risks.[105] During shortages, these policies emphasize equitable access, with ethical guidelines ensuring that blood type barriers do not disproportionately disadvantage underserved groups, as outlined in principles promoting utility, justice, and respect for persons.[106] Reports highlight ongoing inequities, such as longer wait times for rare blood types in minority populations, underscoring the need for allocation models that address systemic disparities in organ availability.[107]Prenatal blood typing raises ethical issues, particularly regarding RhoGAM (Rh immune globulin) administration to prevent hemolytic disease of the newborn (HDN) in Rh-incompatible pregnancies. Decisions on RhoGAM use involve balancing maternal and fetal risks, with guidelines recommending targeted administration based on fetal RhD genotyping to avoid unnecessary exposure, especially amid ongoing shortages as of 2025.[108][109] Non-directive counseling is essential in these scenarios, providing expectant parents with unbiased information on HDN risks, testing options, and interventions without influencing reproductive choices, in line with ethical standards for prenatal genetic counseling.[110]Informed consent for RhoGAM must address potential long-term effects, ensuring autonomy in high-stakes decisions.Global disparities in blood typing access persist, with developing countries facing low coverage due to limited infrastructure and resources, hindering safe transfusions and maternal care.[111]The World Health Organization (WHO) supports national blood systems to enhance availability, aiming to reduce inequities through strategies like the 2025–2028 Global Health Strategy, which prioritizes equitable health services in low-resource settings.[112] Genetic studies on blood types in indigenous or developing populations have sparked biopiracy concerns, where samples are collected without adequate benefit-sharing or consent, potentially exploiting communities for commercial gain.[113] International frameworks, such as the Convention on Biological Diversity, address these by mandating prior informed consent and equitable partnerships in genetic research.[114]