Rh and Other Blood Group Systems

Protein blood group antigens are inherited structures of the red-cell membrane. Their genes, carrier proteins, population frequencies, and susceptibility to reagent treatment help explain a patient’s serologic pattern. Antibody significance then depends on thermal range, immunoglobulin class, complement activation, antigen density, prior exposure, and the survival of transfused cells.

The Rh system

Genes, proteins, and nomenclature

The Rh system, ISBT 004, contains 56 recognized antigens. Its five principal antigens are D, C, c, E, and e. RHD encodes RhD, while RHCE encodes one of four RhCE proteins carrying C or c together with E or e. The two genes lie close together on chromosome 1 and usually travel as a haplotype. RHAG on chromosome 6 encodes the associated glycoprotein required for normal Rh expression. RhD and RhCE are homologous, nonglycosylated 417-amino-acid proteins with 12 membrane-spanning domains.^1,2,3

Four nomenclatures describe the same common antigens and haplotypes:

The lowercase d records the absence of D. The common Wiener mappings are R₁ = DCe, R₂ = DcE, R₀ = Dce, R_z = DCE, r = dce, r′ = dCe, r″ = dcE, and r^y = dCE. Gene symbols are italicized, protein and antigen symbols remain in plain type, and antibody names carry the anti- prefix.

Routine typing establishes a phenotype. A D+C+E−c+e+ phenotype may arise from DCe/dce or another haplotype pair that supplies the same five antigens. Population frequencies can support a probable genotype, while molecular testing can resolve an inherited allele or RHD zygosity when that result changes laboratory interpretation.¹

Population haplotype frequencies also guide donor recruitment. The R₀ (Dce) haplotype is more frequent among donors with African ancestry, so targeted recruitment can improve access to C-negative, E-negative units when D-positive support is appropriate. Rare-unit searches combine validated frequency data with the patient’s complete antibody profile.¹

D expression and variants

A complete RHD deletion is a common molecular basis for D-negative status in people of European ancestry. Other populations include RHD pseudogenes, RHD-RHCE hybrid alleles, and DEL alleles at different frequencies. The serologic label weak D therefore describes a reaction pattern with more than one possible molecular cause.^1,3

For Whole Blood donor labeling, 21 CFR 640.5(c) requires an initially negative anti-D result to receive additional testing that includes weak D. The product may be labeled Rh negative only when the further testing is negative. Either a positive direct anti-D test or a positive weak-D test supports Rh-positive labeling.⁴

Transfusion and obstetric Rh immune globulin decisions for recipients must distinguish serologic reactivity from genotype. The AABB and CAP joint statement recommends RHD genotyping for a pregnant patient or other female of childbearing potential with a serologic weak D phenotype. Recipients with weak D types 1, 2, or 3 can receive D-positive red cells, and these genotypes alone create no indication for Rh immune globulin. Other genotypes follow the risk assigned to that variant and the laboratory’s validated policy.⁵ A recipient with a partial D variant capable of anti-D formation is managed as D negative and generally receives D-negative red cells.^1,5

Rh antibodies and related phenotypes

Rh alloantibodies are usually immune IgG antibodies that react best at 37 °C and the antiglobulin phase. They commonly show dosage and enhanced reactions with papain- or ficin-treated cells. They usually cause extravascular red-cell clearance and can cause hemolytic transfusion reactions and hemolytic disease of the fetus and newborn. Among the common Rh antigens, the usual immunogenicity order is D, c, E, C, then e.¹

Anti-G can resemble anti-D plus anti-C because G is present on nearly all D-positive cells and all C-positive cells. The distinction changes Rh immune globulin eligibility when a pregnant patient has anti-G without anti-D. Transfusion support for anti-G uses D-negative, C-negative red cells. The f antigen is a compound antigen expressed when c and e occur on the same RhCE protein, so anti-f can resolve haplotype arrangements. For example, it reacts with cells carrying the dce haplotype, as in DCE/dce, while its absence of reactivity with DcE/DCe identifies c and e on opposing haplotypes.¹

Rh_null red cells lack all Rh antigens. An RHAG defect produces the regulator type, while inactive Rh structural genes can produce the amorph type. Loss of the Rh complex changes membrane stability and can cause a compensated hemolytic anemia with stomatocytes. A patient who forms anti-Rh17 against the high-prevalence Rh complex may require Rh_null or other exceptionally rare compatible blood.¹

Rh_mod results from partial RHAG function. All Rh antigens are weakly expressed, and the red-cell membrane changes and compensated hemolysis are generally milder than in Rh_null. Broadly weakened Rh typing can therefore reflect an Rh-complex expression defect that warrants reference-laboratory investigation.¹

MNS, Kell, Kidd, Duffy, and Lutheran

These systems account for many non-ABO/Rh antibodies found during pretransfusion testing. The carrier protein predicts several useful laboratory properties.

MNS

M and N are carried on glycophorin A, encoded by GYPA. S, s, and the high-prevalence U antigen are carried mainly on glycophorin B, encoded by GYPB. Homology between these genes permits crossing over and gene conversion, producing hybrid glycophorins, low-prevalence antigens, and phenotypes lacking a high-prevalence antigen.^1,2

Anti-M and anti-N are commonly cold-reactive IgM antibodies and show dosage. Papain and ficin destroy their target structures. Reactivity at 37 °C or in the antiglobulin phase may indicate clinical significance. Anti-S, anti-s, and anti-U are commonly IgG and can cause hemolytic transfusion reactions and hemolytic disease of the fetus and newborn. The S−s−U− phenotype occurs most often in people with African ancestry and creates a rare-unit requirement when anti-U is present.^1,2

Kell and Kx

The Kell system, ISBT 006, contains 38 recognized antigens. The KEL glycoprotein carries K, k, and other antithetical antigen pairs. K is a strong immunogen, and anti-K is usually immune IgG reactive at the antiglobulin phase. Anti-K can cause severe transfusion reactions and can suppress fetal erythropoiesis because Kell is expressed on erythroid precursors.^1,2

Dithiothreitol and other sulfhydryl reagents disrupt Kell antigens. This property supports selected antibody investigations, although the reagent also destroys or weakens additional antigens and every interpretation requires appropriate controls. A K₀ person lacks Kell-system antigens and can form anti-Ku against an antigen present on almost all ordinary red cells.^1,2

The Kx antigen belongs to the XK system and supports normal Kell expression. An XK variant can produce the X-linked McLeod phenotype, with absent Kx, weakened Kell antigens, acanthocytes, and reduced red-cell survival. Compatible support can require McLeod-phenotype blood.^1,2

Kidd

The Kidd glycoprotein, encoded by SLC14A1, transports urea and carries Jk^a, Jk^b, and Jk3. Kidd antibodies often show dosage, bind complement, and react more strongly with enzyme-treated cells. Their concentration can fall below routine detection and rise rapidly after re-exposure. This evanescence and anamnestic response make anti-Jk^a and anti-Jk^b classic causes of delayed hemolytic transfusion reactions.^1,2

Jk(a−b−) cells lack the Kidd urea transporter and resist rapid lysis in 2 mol/L urea. The phenotype is rare overall and occurs at a higher frequency in some Polynesian populations. A person with the null phenotype can form anti-Jk3 and require Jk(a−b−) units.^1,2

Duffy

The Duffy glycoprotein is atypical chemokine receptor 1, encoded by ACKR1, and carries Fy^a, Fy^b, and other Duffy antigens. Anti-Fy^a and anti-Fy^b are usually IgG, react at the antiglobulin phase, and can cause acute or delayed hemolytic transfusion reactions and hemolytic disease of the fetus and newborn. Papain and ficin destroy Duffy antigen reactivity.^1,2

An ACKR1 promoter variant common in people with African ancestry silences erythroid expression and produces Fy(a−b−) red cells while retaining expression in other tissues. These individuals rarely form anti-Fy^b because expression in other tissues supports immune tolerance. A true Duffy-null allele abolishes expression in all tissues and permits anti-Fy^b formation, so molecular context changes antibody risk.^1,2

Erythroid ACKR1 silencing markedly reduces susceptibility to Plasmodium vivax. Infection remains possible. A 2024 Ethiopian study found P. vivax in Duffy-negative participants, with infection substantially more frequent among Duffy-positive participants.⁶

Lutheran

The BCAM adhesion glycoprotein carries Lutheran antigens. Lu^a has low prevalence and Lu^b has high prevalence. Anti-Lu^a is commonly a naturally occurring IgM antibody with a characteristic loose, mixed-field appearance. Anti-Lu^b is usually immune IgG and may shorten transfused-cell survival.^1,2

A recessive BCAM null phenotype lacks Lutheran antigens and can permit anti-Lu3 formation. A dominant KLF1 regulator variant produces the In(Lu) phenotype with markedly weakened Lutheran and selected other antigens. The two mechanisms create different antibody risks.^1,2

Reagent effects and antibody patterns

Enzyme or chemical treatment changes multiple antigens at once. Treated cells therefore provide one line of evidence within a controlled antibody investigation.^1,2

Other red-cell systems and antigen prevalence

Selected systems provide recognizable clues when common panel interpretations fail:^1,2

ISBT places genetically assigned antigens in numbered blood group systems. Unassigned antigens with a prevalence below 1% enter the 700 series, and unassigned antigens with a prevalence above 90% enter the 901 series.²

An antibody to a low-prevalence antigen may be absent from the routine screen because the reagent cells lack its target, then appear when a donor crossmatch is incompatible. An antibody to a high-prevalence antigen commonly reacts with nearly every panel cell while the autocontrol remains negative. The latter pattern prompts review of reagent-cell exclusions, patient ancestry and transfusion history, null phenotypes, molecular testing, and rare-donor resources. Final component selection follows the identified antibody, its clinical significance, and the laboratory’s validated procedure.^1,2

HLA and human platelet antigens

HLA organization and inheritance

The human leukocyte antigen region lies within the major histocompatibility complex on chromosome 6. Mature red cells lack HLA class I and II molecules, while platelets express HLA class I. Class I and class II genes encode antigen-presenting molecules; the class III region contains complement and other immune-response genes.¹

Closely linked HLA alleles inherited on one chromosome form a haplotype. Each full sibling has an approximate 25% probability of sharing both parental haplotypes, a 50% probability of sharing one, and a 25% probability of sharing neither. Recombination within the region and population linkage disequilibrium affect the observed allele combinations.¹

An HLA allele name begins with the locus, an asterisk, and colon-separated fields. In HLA-B*27:05, B identifies the locus, 27 identifies the allele group associated with a serologic family, and 05 distinguishes a specific protein sequence. Additional fields can record synonymous or noncoding sequence differences. The WHO Nomenclature Committee for Factors of the HLA System assigns official names, and the IPD-IMGT/HLA Database maintains the current sequence and nomenclature records. Release 3.65 was issued in July 2026.⁷

Pregnancy, transfusion, and transplantation can stimulate HLA alloantibodies. In transfusion medicine, these antibodies are central to immune platelet refractoriness and selection of HLA-matched or crossmatch-compatible platelets. HLA typing and antibody testing also support donor-recipient matching and transplant risk assessment.¹

Human platelet antigens

Human platelet antigens are polymorphisms on platelet membrane glycoproteins and are classified separately from HLA. HPA-1a and HPA-1b, for example, are carried on integrin β3, historically called glycoprotein IIIa. An HPA alloantibody can cause fetal and neonatal alloimmune thrombocytopenia, post-transfusion purpura, or platelet refractoriness. The ISBT Platelet Immunology Nomenclature Committee assigns HPA names, and the Versiti HPA Gene Database maintains the current antigen and gene record.^1,8

A poor post-transfusion platelet-count increment may reflect HLA antibody, HPA antibody, or a nonimmune cause. The distinction directs the subsequent antibody test and the choice of compatible platelet support.^1,8

Immunogenicity and compatible-unit availability

An antigen is a molecular structure that an antibody or B-cell receptor can bind specifically, or that a T-cell receptor can recognize after appropriate presentation. An immunogen is an antigen that induces an immune response. A hapten is a small molecule that becomes immunogenic after conjugation to a carrier protein. Immunogenicity depends on structural difference from the recipient’s own proteins, antigen density and accessibility, dose and route of exposure, inflammatory context, and the recipient’s immune-response genes.¹

Classic calculated estimates of the probability that an antigen-negative transfusion recipient will form the corresponding antibody after a single exposure to antigen-positive red cells rank several antigens as follows:⁹

These values are historical model estimates derived from antibody occurrence and antigen exposure. Study population, number of exposures, pregnancy history, testing sensitivity, antibody evanescence, and recipient biology change the observed rate. Their durable teaching value is the comparative order: D is highly immunogenic, K follows among common non-ABO antigens, and several antibodies with lower formation rates remain clinically important once present.⁹

Antigen prevalence answers a separate compatibility question. K is absent from most donor units, so K-negative units are usually obtainable. High-prevalence k is present on nearly all units, making anti-k support difficult despite its lower immunogenicity. The same principle applies to anti-U, anti-Ku, anti-Jk3, anti-Lu3, anti-Vel, and antibodies against other high-prevalence antigens. An antibody’s clinical significance determines the required compatibility; prevalence determines how far the search may need to extend.^1,2

References

1. Bloch EM, Campbell-Lee S, McKenna DH Jr, et al, eds. Technical Manual. 22nd ed. AABB; 2026. Accessed August 27, 2026.
2. International Society of Blood Transfusion. ISBT Blood Group Database, August 2026 release, version 18. Updated August 3, 2026. Accessed August 27, 2026.
3. International Society of Blood Transfusion Working Party on Red Cell Immunogenetics and Blood Group Terminology. RHD blood group alleles. Updated September 30, 2022. Accessed August 27, 2026.
4. Determination of the Rh factors, 21 CFR § 640.5(c) (2026). Accessed August 27, 2026.
5. AABB, College of American Pathologists, American College of Obstetricians and Gynecologists, et al. Joint Statement on Phasing-In RHD Genotyping for Pregnant Women and Other Females of Childbearing Potential With a Serologic Weak D Phenotype. 2015. Accessed August 27, 2026.
6. Bradley L, Yewhalaw D, Hemming-Schroeder E, et al. Epidemiology of Plasmodium vivax in Duffy negatives and Duffy positives from community and health centre collections in Ethiopia. Malar J. 2024;23:76. doi:10.1186/s12936-024-04895-1.
7. IPD-IMGT/HLA Database. Release 3.65 version report. July 2026. Accessed August 27, 2026.
8. Versiti. Human Platelet Antigen Gene Database. Accessed August 27, 2026.
9. Giblett ER. A critique of the theoretical hazard of inter vs. intra-racial transfusion. Transfusion. 1961;1(4):233-238. doi:10.1111/j.1537-2995.1961.tb00048.x.