10.2 The Rh Blood Group System, Weak D Testing, and Other Major Blood Group Systems

The Rh Blood Group System, Weak D Testing, and Other Major Blood Group Systems

The Rh blood group system is the second most clinically significant system in transfusion medicine after ABO. Unlike ABO antibodies, which are naturally occurring isohemagglutinins present in all immunocompetent individuals, Rh antibodies and antibodies to other major blood group systems are immune-mediated alloantibodies produced exclusively following foreign red cell exposure via transfusion or feto-maternal hemorrhage. A detailed understanding of Rh genetics, nomenclature, weak D expression, and the distinctive serologic behaviors of non-ABO blood group systems—such as Kell, Duffy, Kidd, MNS, Lewis, P, and I—is indispensable for pre-transfusion compatibility testing, antibody identification, and the prevention of hemolytic transfusion reactions and Hemolytic Disease of the Fetus and Newborn (HDFN).

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1. Molecular Genetics and Biochemistry of the Rh Blood Group System

                             ┌─────────────────────────────────────────────────────────┐
                             │           Rh System Genetics (Chromosome 1p36)          │
                             └────────────────────────────┬────────────────────────────┘
                                                          │
                            ┌─────────────────────────────┴─────────────────────────────┐
                            ▼                                                           ▼
                        RHD Gene                                                    RHCE Gene
                (Codes for RhD protein)                                     (Codes for RhCE protein)
                            │                                                           │
               • Expresses D antigen (Rh+)                                 • Alternative splicing produces:
               • Complete deletion in Caucasian Rh-                          Ce, cE, ce, or CE antigens
               • Inactive pseudogene in African Rh-                        • Both are hydrophobic 417-aa
               • Point mutations in Asian Rh-                                transmembrane proteins with 12 loops

Chromosomal Organization and Protein Structure

The Rh blood group antigens are encoded by two tightly linked, homologous genes located in a head-to-head orientation on chromosome $1p36.11$:

• $RHD$ Gene: Encodes the RhD protein, which confers Rh-positivity. In Caucasian Rh-negative individuals, the Rh-negative phenotype is caused by a complete homozygous deletion of the entire $RHD$ gene ($del/del$). In individuals of African descent, the Rh-negative phenotype is predominantly caused by an inactive $RHD$ pseudogene ($RHD\psi$) containing a 37-base pair duplication in exon 4 and a nonsense mutation that prevents functional protein expression. In Asian populations, Rh-negativity is frequently due to specific point mutations or partial $RHD\text{-}RHCE$ hybrid genes.
• $RHCE$ Gene: Encodes the RhCE protein, which carries the $C$ or $c$ antigens along with the $E$ or $e$ antigens via alternative splicing and specific amino acid substitutions. Serine at position 103 corresponds to $C$, whereas proline corresponds to $c$; proline at position 226 corresponds to $E$, whereas alanine corresponds to $e$.
• Biochemical Properties: Unlike ABO antigens, which are carbohydrate residues attached to glycoproteins and glycolipids, Rh antigens are non-glycosylated, hydrophobic transmembrane proteins comprising 417 amino acids that traverse the erythrocyte phospholipid bilayer 12 times, with both the amino ($NH_2$) and carboxy ($COOH$) termini located intracellularly. Rh proteins complex with the Rh-associated glycoprotein ($RhAG$) on chromosome 6 to maintain red cell membrane skeletal integrity and facilitate ammonium transport.
• The $Rh_{null}$ Syndrome:
  • Caused either by the amorphic mutation (homozygous mutation/deletion of $RHCE$ in an $RHD$-deleted background) or the regulator mutation (homozygous defect in the $RHAG$ gene on chromosome 6, preventing membrane assembly of Rh proteins).
  • Erythrocytes completely lack all Rh antigens ($D, C, c, E, e$) and LW antigens.
  • Clinically, $Rh_{null}$ red cells display structural membrane instability, presenting with stomatocytosis, spherocytosis, increased osmotic fragility, and chronic compensated extravascular hemolytic anemia. If alloimmunized, $Rh_{null}$ patients produce an all-encompassing anti-Rh29 (anti-total Rh), making them compatible strictly with blood from other $Rh_{null}$ donors.

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2. Rh Nomenclature Systems and Genotype Conversions

Four distinct nomenclatures describe Rh phenotypes and genotypes: Fisher-Race (CDE), Wiener ($Rh\text{-}Hr$), Rosenfield numeric, and the International Society of Blood Transfusion (ISBT) numeric classification.

                          ┌─────────────────────────────────────────────────────────────────┐
                          │               Wiener to Fisher-Race Conversions                 │
                          └───────────────────────────────┬─────────────────────────────────┘
                                                          │
               ┌──────────────────────────────┬───────────┴───────────┬──────────────────────────────┐
               ▼                              ▼                       ▼                              ▼
           R1 = CDe                       R2 = cDE                R0 = cDe                       Rz = CDE
         (Rh-positive)                  (Rh-positive)           (Rh-positive)                  (Rh-positive)
               ▲                              ▲                       ▲                              ▲
               │                              │                       │                              │
           r' = Cde                       r'' = cdE                r = ce                        ry = CdE
         (Rh-negative)                  (Rh-negative)           (Rh-negative)                  (Rh-negative)

The Fisher-Race vs. Wiener Systems

• Fisher-Race (CDE): Hypothesized that three closely linked genetic loci encode five major antigens: $D$, $C$, $c$, $E$, and $e$. The absence of D is designated by a lowercase "d" (denoting a silent allele or deletion, not an actual antigen).
• Wiener ($Rh\text{-}Hr$): Hypothesized that a single gene locus produces an agglutinogen composed of multiple antigenic "factors." Wiener shorthand symbols use uppercase $R$ when the $D$ antigen is present and lowercase $r$ when $D$ is absent:
  • $R^1$ ($CDe$) and $r'$ ($Cde$): Contain $C$ (denoted by superscript 1 or single prime $'$).
  • $R^2$ ($cDE$) and $r''$ ($cdE$): Contain $E$ (denoted by superscript 2 or double prime $''$).
  • $R^0$ ($cDe$) and $r$ ($ce$): Contain neither $C$ nor $E$ (contain $c$ and $e$; basic form).
  • $R^z$ ($CDE$) and $r^y$ ($CdE$): Contain both $C$ and $E$ (rare alleles; denoted by $z$ or $y$).

Rosenfield and ISBT Systems

• Rosenfield Numeric System: Assigns numbers based on discovery order without genetic implications: $Rh1 = D$, $Rh2 = C$, $Rh3 = E$, $Rh4 = c$, $Rh5 = e$. A minus sign indicates absence. For example, phenotype $C+c+D+E-e+$ is written as $Rh: 1, 2, -3, 4, 5$.
• ISBT System: Designates the Rh system as collection 004, with each antigen given a permanent six-digit number ($004001 = D$, $004002 = C$, $004003 = E$, $004004 = c$, $004005 = e$).

Population Frequencies and Immunogenicity

• Haplotype Frequencies:
  • In Asian / Filipino populations: $R^1$ ($CDe$) is predominant ($\approx 70\%$), followed by $R^2$ ($cDE$, $\approx 20\%$) and $R^0$ ($cDe$, $\approx 5\%$). The D-negative phenotype is rare in Filipinos and East Asians (well under 1% of people, compared with about 15% of Caucasians), because the $r$ ($ce$) haplotype is uncommon in these populations.
  • In African populations: $R^0$ ($cDe$) is the most common ($\approx 44\%$), followed by $r$ ($\approx 26\%$) and $R^1$ ($\approx 21\%$).
  • In Caucasian populations: $R^1$ ($CDe$, $\approx 42\%$) is most common, followed by $r$ ($ce$, $\approx 37\%$) and $R^2$ ($cDE$, $\approx 14\%$).
• Immunogenicity Ranking: The ability of foreign Rh antigens to stimulate antibody formation in antigen-negative recipients follows a strict hierarchy: $\mathbf{D > c > E > C > e}$
  • More than 50% to 85% of D-negative individuals transfused with a single unit of D-positive red cells develop anti-D alloantibodies. The $c$ antigen is the second most immunogenic Rh antigen and a frequent cause of severe delayed transfusion reactions and HDFN.

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3. Weak D Phenotypes and Transfusion Policies

                                  ┌──────────────────────────────────────────────┐
                                  │           Weak D Expression Mechanisms       │
                                  └──────────────────────┬───────────────────────┘
                                                         │
         ┌───────────────────────────────────────────────┼───────────────────────────────────────────────┐
         ▼                                               ▼                                               ▼
  ┌──────────────┐                                ┌──────────────┐                                ┌──────────────┐
  │ Genetic Weak │                                │ Position /   │                                │  Partial D   │
  │      D       │                                │   C-trans    │                                │  (D Mosaic)  │
  └──────┬───────┘                                └──────┬───────┘                                └──────┬───────┘
         │                                               │                                               │
  • Point mutations in trans-                     • Steric hindrance when                         • Missing D epitopes
    membrane / intracellular                        C allele is trans to D                          due to gene conversion
  • Quantitative reduction                          (e.g., Dce / dCe)                               (RHD-RHCE hybrid)
    (<5,000 sites vs 30,000)                      • Structurally complete                         • Qualitative defect
  • Structurally complete                         • Cannot form allo-anti-D                       • CAN FORM TRUE
  • Cannot form allo-anti-D                                                                         ALLO-ANTI-D!

Mechanisms of Weak D Expression

When tested at immediate-spin with commercial monoclonal anti-D, some red cells fail to agglutinate directly ($0$) or yield weak reactions ($<= 2+$), but agglutinate strongly ($3+\text{ to }4+$) following $37^\circ\text{C}$ incubation and the addition of Antihuman Globulin (AHG). This weak D phenotype arises via three distinct mechanisms:

1. Genetic Weak D: Point mutations within the cytoplasmic or transmembrane domains of the $RHD$ gene reduce the efficiency of RhD protein assembly and insertion into the red cell membrane. Antigen site density drops from a normal level of $10,000\text{--}33,000$ sites/cell to fewer than $1,000\text{--}5,000$ sites/cell. Crucial point: The D antigen is structurally complete (all epitopes are present); these individuals do NOT produce allo-anti-D upon exposure to normal D-positive red cells.
2. Position Effect / $C\text{-}trans$ Effect: Occurs when the $C$ allele is inherited in trans position to the $D$ allele on the opposite homologous chromosome (for example, genotype $Dce/dCe$, or $R^0/r'$). The spatial conformation of the $C$ protein in trans sterically hinders RhD antigen expression. The D epitopes are structurally intact, and these individuals cannot produce allo-anti-D.
3. Partial D (D Mosaic / D Variant): Caused by hybrid gene formation between $RHD$ and $RHCE$, where portions of the $RHD$ gene are replaced by homologous segments of $RHCE$. The resulting RhD protein lacks one or more specific outer exofacial epitopes (from the complex of over 30 distinct D epitopes, epD1 through epD30). Vital Clinical Consequence: Partial D individuals CAN produce a true allo-anti-D directed against the specific missing epitopes if transfused with conventional, complete D-positive red blood cells!

Weak D Testing Protocol and Disparate Clinical Policies

Weak D testing requires incubating patient red cells with commercial anti-D reagent at $37^\circ\text{C}$ for 15 to 30 minutes, performing 3 to 4 saline washes, and adding Antihuman Globulin (AHG / Coombs reagent).

                              ┌─────────────────────────────────────────────────────────┐
                              │         Weak D Testing Clinical Policies Mandate        │
                              └────────────────────────────┬────────────────────────────┘
                                                           │
                            ┌──────────────────────────────┴─────────────────────────────┐
                            ▼                                                            ▼
                   Blood Donors & Neonates                                      Transfusion Recipients
  • Weak D testing via IAT is MANDATORY.                       • Weak D testing via IAT is NOT RECOMMENDED.
  • If weak D test is POSITIVE:                                • If immediate-spin anti-D is NEGATIVE:
    Unit is labeled Rh-POSITIVE.                                 Patient is classified as Rh-NEGATIVE.
  • Rationale: Weak D red cells can stimulate                  • Rationale: Protects Partial D patients from
    primary anti-D alloimmunization in an                        receiving D-positive RBCs and developing
    Rh-negative transfusion recipient.                           potentially fatal allo-anti-D.

• Blood Donors: MANDATORY. All donor units that type negative at immediate spin with anti-D must undergo weak D testing through the AHG phase. If positive at AHG, the unit is labeled Rh-Positive because the transfused weak D red cells can easily elicit primary anti-D alloimmunization in an Rh-negative recipient.
• Neonates of Rh-Negative Mothers: MANDATORY. Cord blood from infants born to Rh-negative mothers must undergo weak D testing. If the infant is weak D positive, the mother must receive Rh Immune Globulin (RhIg / RhoGAM) prophylaxis within 72 hours of delivery to prevent maternal anti-D formation.
• Transfusion Recipients (Patients): NOT RECOMMENDED / NOT REQUIRED. Patients whose red cells are negative at immediate spin are classified as Rh-Negative and must receive Rh-negative blood components. Performing weak D testing on recipients is dangerous: if a Partial D patient were tested and labeled Rh-positive, they might be transfused with D-positive blood, resulting in severe allo-anti-D immunization.

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4. Other Major Clinically Significant Blood Group Systems

                                  ┌──────────────────────────────────────────────┐
                                  │      Major Non-ABO Blood Group Systems       │
                                  └──────────────────────┬───────────────────────┘
                                                         │
         ┌──────────────────────────┬────────────────────┴──────────────┬──────────────────────────┐
         ▼                          ▼                                   ▼                          ▼
  ┌──────────────┐           ┌──────────────┐                    ┌──────────────┐           ┌──────────────┐
  │ Kell System  │           │ Duffy System │                    │ Kidd System  │           │  MNS System  │
  └──────┬───────┘           └──────┬───────┘                    └──────┬───────┘           └──────┬───────┘
         │                          │                                   │                          │
  • K antigen highly         • Fya, Fyb destroyed                • Jka, Jkb show dosage,     • M, N on GPA
    immunogenic                by enzymes                          enhanced by enzymes         (destroyed by enzymes)
  • Destroyed by DTT         • Plasmodium vivax                  • Traitor antibody: drops   • S, s on GPB
  • Suppresses fetal           receptor; Fy(a-b-)                  below detection; severe     (enzyme resistant)
    erythropoiesis (BFU-E)     confers resistance                  delayed HTR (DHTR)        • Anti-S, s: warm IgG

The Kell System (ISBT 006)

• Antigens: Consists of high- and low-prevalence antithetical pairs: $K$ (KEL1 / Kell, ~9% prevalence) and $k$ (KEL2 / Cellano, 99.8% prevalence), $Kp^a$ (low) / $Kp^b$ (high), and $Js^a$ (low) / $Js^b$ (high).
• Biochemical Nature: A large single-pass transmembrane glycoprotein linked by a single disulfide bond to the Kx protein (encoded on the X chromosome).
• Reagent Sensitivity: Kell antigens depend on intact disulfide bonds; they are completely destroyed by sulfhydryl reagents (such as $0.2\text{ M}$ dithiothreitol [DTT], 2-mercaptoethanol [2-ME], and AET), but are resistant to proteolytic enzymes (ficin and papain).
• Immunogenicity & Clinical Impact: The $K$ antigen is the most immunogenic non-ABO red cell antigen. Anti-K is a clinically significant warm-reactive IgG antibody. In pregnancy, maternal anti-K crosses the placenta and induces severe fetal anemia not merely by peripheral splenic hemolysis, but uniquely by binding and destroying early erythroid progenitor cells ($BFU\text{-}E$ and $CFU\text{-}E$), causing severe hyporegenerative fetal anemia with remarkably low reticulocyte counts.
• The McLeod Phenotype: Rare X-linked condition characterized by absence of the Kx protein and severe depression of Kell antigen expression. Patients suffer from acanthocytic hemolytic anemia, elevated serum creatine kinase (CK), progressive late-onset sensorimotor neuropathy, and a strong association with X-linked Chronic Granulomatous Disease (CGD).

The Duffy System (ISBT 008)

• Antigens: The primary antithetical antigens are $Fy^a$ and $Fy^b$, carried on the Duffy Antigen Receptor for Chemokines (DARC / ACKR1), a seven-transmembrane glycoprotein.
• Enzyme Sensitivity: Duffy antigens are completely destroyed by proteolytic enzymes (ficin, papain, and bromelin), which cleave their extracellular N-terminal domain.
• Malaria Resistance: The DARC glycoprotein serves as the obligate erythrocyte receptor for merozoite invasion by the malaria parasite Plasmodium vivax (and Plasmodium knowlesi). The $Fy(a-b-)$ phenotype, resulting from a single nucleotide mutation in the erythroid promoter region ($GATA\text{-}1$ box) that silences gene expression exclusively in erythrocytes, is present in over 68% of Black Africans, conferring near-total genetic resistance to P. vivax malaria.
• Antibodies: Anti-$Fy^a$ and anti-$Fy^b$ are warm IgG antibodies that react optimally at the AHG phase, do not typically bind complement, and cause acute/delayed hemolytic transfusion reactions and mild HDFN.

The Kidd System (ISBT 009)

• Antigens: $Jk^a$ and $Jk^b$, carried on the red blood cell urea transporter protein (UT-B / SLC14A1).
• Dosage and Enzyme Enhancement: Kidd antigens exhibit prominent dosage (anti-$Jk^a$ reacts much more strongly with homozygous $Jk(a+b-)$ cells than with heterozygous $Jk(a+b+)$ cells) and are significantly enhanced by proteolytic enzymes (ficin and papain).
• Clinical Notoriety: Delayed Hemolytic Transfusion Reactions (DHTR): Known in immunohematology as "the sneaky or traitor antibody." Following primary exposure, anti-$Jk^a$ and anti-$Jk^b$ titers rapidly decline below the analytical detection limits of routine pre-transfusion antibody screening assays. When the patient is later transfused with antigen-positive blood, an aggressive anamnestic secondary immune response produces high antibody titers within 3 to 7 days. Kidd antibodies avidly bind complement, triggering severe acute or delayed extravascular and intravascular hemolytic transfusion reactions.
• $Jk(a-b-)$ Phenotype: Rare phenotype (found predominantly in Polynesians and Finns); red blood cells lack the urea transporter and resist osmotic lysis in $2\text{ M}$ urea for up to 30 minutes (normal red cells lyse within 1 minute).

The MNS System (ISBT 002)

• Antigens $M$ and $N$: Located on the outer terminal domain of Glycophorin A (GPA), rich in sialic acid. Destroyed completely by proteolytic enzymes (ficin and papain). Anti-M and anti-N are typically naturally occurring, cold-reactive IgM antibodies that exhibit dosage and are clinically insignificant unless they react at $37^\circ\text{C}$ or AHG.
• Antigens $S$, $s$, and $U$: Located on Glycophorin B (GPB). The $U$ antigen (Universal) is a high-frequency antigen present in >99.9% of the population, located near the membrane. Anti-S, anti-s, and anti-U are warm-reactive IgG antibodies that cause severe acute and delayed HTR and HDFN.

The Lewis System (ISBT 007)

• Antigens $Le^a$ and $Le^b$: Lewis antigens are not synthesized by erythrocytes. Instead, soluble glycolipids synthesized by intestinal mucosal epithelial cells are secreted into plasma and passively adsorbed onto the erythrocyte membrane.
• Pregnancy Loss: During pregnancy, elevated maternal plasma lipoprotein levels absorb circulating Lewis glycolipids, causing pregnant women to transiently type as $Le(a-b-)$. They frequently develop transient, benign Lewis antibodies that disappear following delivery.
• Clinical Features: Lewis antibodies (anti-$Le^a$ and anti-$Le^b$) are naturally occurring IgM antibodies that bind complement at room temperature. They never cause HDFN because: (1) maternal IgM does not cross the placenta, and (2) fetal red cells lack Lewis antigens at birth (type as $Le(a-b-)$). Lewis antibodies can be neutralized in vitro by secretor saliva or commercial Lewis substance.

The P System (ISBT 003) and Donath-Landsteiner Hemolysin

• Anti-$P_1$: A common, naturally occurring cold IgM antibody that can be neutralized in vitro by hydatid cyst fluid (Echinococcus granulosus) or pigeon egg white.
• Auto-Anti-P (The Donath-Landsteiner Antibody): Pathognomonic marker of Paroxysmal Cold Hemoglobinuria (PCH), a severe autoimmune hemolytic anemia seen following viral infections in children or tertiary syphilis. It is a biphasic IgG autohemolysin: it binds to patient erythrocytes and fixes early complement components ($C1\text{--}C4$) at reduced temperatures ($4^\circ\text{C}$) in the peripheral capillary beds; upon rewarming to $37^\circ\text{C}$ in the central circulation, the terminal complement cascade ($C5b\text{-}9$) activates, triggering sudden, massive intravascular hemolysis and dark red-brown hemoglobinuria.

The I System (ISBT 027)

• Developmental Genetics: The $i$ antigen represents straight, unbranched carbohydrate chains abundant on neonatal cord blood erythrocytes ("i = infant"). Over the first two years of life, the enzyme branching glycosyltransferase converts linear $i$ chains into complex branched $I$ antigen structures found on adult erythrocytes.
• Cold Autoantibodies: High-titer auto-anti-I (IgM) is strongly associated with atypical pneumonia caused by Mycoplasma pneumoniae, causing Cold Agglutinin Disease (CAD). Conversely, auto-anti-i is associated with infectious mononucleosis caused by Epstein-Barr Virus (EBV) and Cytomegalovirus (CMV).

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5. Comprehensive Blood Group System Reference Table