Sept.-3: MLT 225 Comprehensive Study Guide to ABO Discrepancies and the Rh Blood Group System Pt. 2

ABO Discrepancies, Technical Artifacts, and Resolution Strategies

Rouleaux Formation vs. True Agglutination:
- Rouleaux appears as pseudo-agglutination due to elevated plasma protein levels causing red blood cells to stack or clump naturally.
- Resolution: Perform a saline wash of the patient red blood cells (washing once or multiple times if necessary).
- Outcome: Protein is washed away during saline washing. If the initial clumping was Rouleaux, true negative reactions will clear to negative (NEG). If true agglutination is present, the agglutination will persist after washing.
ABO Antigen Expression in Secretors vs. Non-Secretors:
- Secretor status affects the presence of soluble ABO blood group antigens in body fluids (such as saliva, semen, or sweat).
- Red cell testing: Regardless of secretor or non-secretor status, ABO antigens are always expressed on red blood cells. Secretor status does not alter routine ABO forward typing on red cells.
- Forensics: Secretor status becomes clinically relevant only in forensic laboratories typing non-blood body fluids.
Rare ABO Discrepancies and Survey Data:
- Evaluation of standard board examination study materials (comprising $379$ reviewed blood bank questions) demonstrates that complex discrepancies like B(A) phenotypes or Acquired B antigens are exceptionally rare.
- Group A with Acquired B Antigen: Occurs primarily in Group $A_1$ individuals following bacterial enzymatic deacetylation of the N-acetylgalactosamine ( $A$ antigen immunodominant sugar).
- Structural alteration: Deacetylation transforms N-acetylgalactosamine into galactosamine, which structurally mimics galactose (the $B$ antigen immunodominant sugar). Consequently, standard anti-B blood bank reagents cross-react with this modified antigen, giving a false appearance of a Group $AB$ phenotype with an acquired B antigen.
Interpretation Rules and Laboratory Documentation:
- Results must always be documented using clear alpha character abbreviations (POS for positive and NEG for negative).
- Plus signs ( $+$ ) and minus signs ( $-$ ) are strictly prohibited on official blood bank interpretation records to eliminate misreadings caused by hurried or poor handwriting.
- Standard designation convention places the D antigen typing status directly after the ABO group (e.g., A POS, O NEG, B POS, AB POS, AB NEG).
Classification and Causes of Weak or Missing ABO Reactions:
- Mixed-Field Reactions: Characterized by small agglutinates against a background of unagglutinated free cells (e.g., a $2+$ mixed-field reaction in forward typing). Frequently seen in Group A or Group B patients who have received recent transfusions of Group O packed red blood cells.
- Subgroups of A: Weak or missing forward typing reactions combined with unexplained extra antibodies in reverse typing often indicate an $A$ subgroup (e.g., $A_2$ subgroup with anti- $A_1$ antibody).
  - Resolution procedure: Test patient red cells with Anti- $A_1$ lectin (Dolichos biflorus) and test patient serum/plasma against $A_2$ reagent red blood cells.
- Missing or Weak Reverse Antibodies: Missing ABO antibodies in serum/plasma reverse typing can stem from multiple physiological, therapeutic, or technical causes:
  - Declining antibody titers (common in neonates and elderly patients).
  - Hypogammaglobulinemia or congenital/acquired immunodeficiency states.
  - Immunosuppressive therapies, including chemotherapy or targeted monoclonal antibody/immunotherapy treatments.
  - Bone marrow or hematopoietic stem cell transplantation: Recipients receiving ABO-mismatched donor marrow (e.g., a Group A recipient receiving Group B marrow) will experience antibody suppression and a gradual transition in red cell antigen phenotype.
    - Full red blood cell engraftment represents the final phase of complete marrow engraftment, taking up to $3\text{ months}$ ( $90\text{ days}$ ) to show complete conversion.
    - Engraftment status is tracked by specialized transplant laboratories using Human Leukocyte Antigen (HLA) markers and CD cluster differentiation markers.
    - Regulatory Compliance: Changing a patient's documented blood type in laboratory information systems following transplantation requires rigorous, dotted-i-and-crossed-t supporting documentation to fulfill Food and Drug Administration (FDA) audit standards and deviation reporting requirements.
  - Technical errors (e.g., missed reagent drops, improper incubation, incorrect cell-to-plasma ratios).

The Rh Control and Spontaneous Agglutination Testing

Clinical Indication for Rh Control Testing:
- Required specifically when a patient forward-types as Group AB, D-positive (yielding positive reactions across all forward channels: Anti-A positive, Anti-B positive, and Anti-D positive, with no negative reaction present to serve as an internal control).
Diagnostic Purpose:
- Serves to rule out spontaneous agglutination of patient red blood cells. Without a negative control reaction, positive results in Anti-A, Anti-B, and Anti-D cannot be confirmed as true antigen-antibody reactions.
Methodologies for Spontaneous Agglutination Verification:
1. Autologous Control (Auto Control):
  - Consists of testing patient serum/plasma against a $3\%$ saline suspension of the patient's own red blood cells.
  - No additional saline is added directly into the test system during setup; maintaining a precise $3\%$ cell suspension prevents false readings caused by incorrect plasma-to-cell ratios.
  - A negative autologous control confirms that the patient's cells are not spontaneously agglutinating in plasma, verifying that forward typing positive reactions are valid.
  - Utilized as the standard protocol in health systems such as University hospitals and Baptist health facilities.
2. Rh-HR Control Reagent:
  - Consists of a low-protein diluent formulation matching the media composition of monoclonal Anti-D reagents without the active antibody.
  - Must yield a completely negative result (NEG).
  - A positive reaction in the Rh-HR control indicates false-positive agglutination caused by hyperproteinemia, Rouleaux, or autoantibodies.
  - Utilized as standard protocol in NORD health facilities.

Overview and Clinical Significance of the Rh System

System Significance and History:
- Discovered in $1939$ , the Rh system is the second most clinically significant blood group system in transfusion medicine, behind the ABO system.
- Detailed in Chapter 6 of standard transfusion medicine texts.
- Comprises over $50$ distinct, genetically related antigens.
The Main Five Rh Antigens:
- Primary focus centers on the five principal antigens: D, C, E, c, and e.
- Antigens are denoted as capital letters (D, C, E) or lowercase letters (c, e).
- Handwriting Convention: When handwriting laboratory records, a distinct horizontal bar or line must be drawn directly above the lowercase c ( $\bar{c}$ ) to prevent misinterpretation between capital C and lowercase c.
Clinical Immunogenicity and Alloantibody Eradication:
- Rh antigens (particularly the D antigen) are potent immunogens. Exposure to the D antigen in a D-negative individual results in antibody formation (anti-D) in approximately $65\%$ of cases.
- Historically, anti-D was responsible for the majority of severe hemolytic transfusion reactions and Hemolytic Disease of the Fetus and Newborn (HDFN).
- Routine administration of Rh Immune Globulin (RhoGAM) during pregnancy in D-negative mothers has virtually eradicated anti-D alloimmunization in females.
- Consequently, contemporary anti-D antibodies are predominantly identified in males or older females who bore children prior to the widespread implementation of RhoGAM prophylaxis.
- Transfusion Considerations for Anti-D:
  - Patients with immune anti-D must strictly receive D-negative red blood cell products.
  - Neutralizing one full unit ( $1\text{ bag}$ ) of packed red blood cells transfused to a D-negative patient would require an extreme dose of $17$ separate injections of RhoGAM (RhIG).
Non-Applicability of Landsteiner's Rule:
- Landsteiner's Rule (predictable presence of reciprocal antibodies in serum when the corresponding antigen is absent) applies only to the ABO system.
- Rh antibodies are never naturally occurring; their production strictly requires exposure to foreign red cell antigens via:
  1. Blood transfusion.
  2. Pregnancy (fetal-maternal hemorrhage).
  3. Intravenous (IV) drug use with shared needles.
  4. Stem cell or bone marrow transplantation introducing foreign antigen profiles.

Historical Discovery and the Landsteiner-Wiener (LW) Antigen System

The Rhesus Macaque Experiment:
- Dr. Karl Landsteiner and Dr. Alexander Wiener immunized rabbits and guinea pigs with red blood cells obtained from Rhesus macaques (Macaca mulatta).
- The serum harvested from these immunized animals agglutinated approximately $85\%$ of human red blood cell samples.
- Landsteiner deduced that $85\%$ of humans possessed an antigen identical to that of the Rhesus monkey, designating it the "Rh factor."
Discovery of the True LW Antigen System:
- Subsequent research revealed that the antibody produced by the rabbits and guinea pigs was not recognizing the human D antigen of the Rh system, but rather a separate, distinct antigen system.
- Out of respect for Dr. Landsteiner's pioneering work, the original Rh name was retained for the D/C/E system, and the newly recognized system was named the Landsteiner-Wiener (LW) antigen system.
Cross-Reactivity and Clinical Relevance:
- The LW antigen is phenotypically linked to the D antigen; D-positive cells express LW antigens strongly, whereas D-negative cells express LW weakly or not at all.
- Anti-LW cross-reacts with D-positive cells, mimicking anti-D or complex ABO discrepancies (such as making a Group O, D-positive individual appear to express unexpected antibodies like anti-B).
- Although specific diagnostic reagents for LW are rare in routine laboratories, an LW-negative patient who forms anti-LW must be transfused with D-negative blood products to ensure compatibility.

Genetics and Nomenclature Systems of the Rh System

Evolution of Rh Inheritance Models:
1. Wiener Theory (Single-Locus / Agglutinogen Theory):
  - Proposed that a single gene locus codes for an agglutinogen containing three factor structures ("10 people living under one roof").
  - Termed the original rhesus macaque antigen as $Rh_0$ .
2. Fisher-Race Theory (Three-Loci Theory):
  - Proposed three closely linked allele loci on a single chromosome: one for D/d, one for C/c (and $C^w$ ), and one for E/e.
  - Gene order specified as DCE (frequently written alphabetically as CDE).
  - No "d" Antigen: There is no antithetical "d" antigen. The symbol "d" or an underscore ( $\_$ ) simply denotes the absence of the D antigen. A heterozygous individual is written as $D\_$ or $D/d$ .
3. Tippett Theory (Two-Loci Theory - Contemporary Model):
  - Discovered by Dr. Tippett and confirmed by the Human Genome Project.
  - Establishes that Rh antigens are encoded by two closely linked structural genes on chromosome 1:
    - $RHD$ : Codes specifically for the presence of the D protein/antigen.
    - $RHCE$ : Codes for the combined C/c and E/e envelope proteins ( $CE$ , $Ce$ , $cE$ , or $ce$ ).
Nomenclature Conversions (Fisher-Race vs. Modified Wiener):
- Fundamental Wiener Conversion Rules:
  - Capital $R$ indicates the presence of the D antigen (D-positive).
  - Lowercase $r$ indicates the absence of the D antigen (D-negative).
  - $0$ (zero) or plain $r$ : Indicates lowercase $c$ and lowercase $e$ ( $ce$ ).
  - $1$ or prime ( $^{\prime}$ ): Indicates capital $C$ and lowercase $e$ ( $Ce$ ).
  - $2$ or double prime ( $^{\prime\prime}$ ): Indicates lowercase $c$ and capital $E$ ( $cE$ ).
  - $z$ (for capital $R$ ) or $y$ (for lowercase $r$ ): Indicates all capital antigens ( $CE$ ).
- Complete Rh Haplotype Cross-Reference:
  - $R_0 = Dce$
  - $R_1 = DCe$
  - $R_2 = DcE$
  - $R_z = DCE$
  - $r = ce$ (or $dce$ )
  - $r^{\prime} = Ce$ (or $dCe$ )
  - $r^{\prime\prime} = cE$ (or $dcE$ )
  - $r_y = CE$ (or $dCE$ )
Additional Nomenclature Systems:
- Rosenfield System: A numerical system assigning numbers based on sequence of discovery (e.g., $\text{Rh1} = D$ , $\text{Rh2} = C$ , $\text{Rh3} = E$ , $\text{Rh4} = c$ , $\text{Rh5} = e$ ).
- ISBT (International Society of Blood Transfusion) System: A standardized, machine-readable numerical system suitable for computer databases, though not practical for routine clinical discussions or genetic Punnett squares.

Secretors: About Secretor Genes

Sese or SeSe are secretor genes that allows the expression of antigens A, B, and/or H in body fluids such as breast milk, saliva, tears, urine, amniotic fluid, bile, exudates, semen and digestive fluid.
~80% of the population has this gene
The Se gene can be found in all body fluids except for cerebral spinal fluid (CSF)
The recessive sese are amorphic and are non-secretor genes
SeSe or Sese controls the expression of the H antigen

Population Genetics, Antigen Frequencies and Transfusion Practice

Transfusion Matching Standards by Region:
- United States Practice: Standard transfusion practice requires matching blood units only for ABO and the D antigen. Antigens C, c, E, and e are not routinely matched for general patient populations, creating opportunities for alloimmunization upon foreign antigen exposure.
- British (NHS) and Canadian Practice: Transfusion services match donors and recipients across all five principal Rh antigens (D, C, E, c, e). This practice virtually eliminates Rh alloantibody formation in patients, though it significantly increases the complexity of managing donor inventory.
Reagent Red Cell Antigen Configurations:
- Commercial screening cell panels utilize fixed Rh genotypes to ensure comprehensive antibody detection:
  - Screening Cell 1: Homozygous $R_1R_1$ ( $DCe/DCe$ ).
  - Screening Cell 2: Homozygous $R_2R_2$ ( $DcE/DcE$ ).
  - Screening Cell 3 (in 3-cell panels): Homozygous $rr$ ( $ce/ce$ ).
Rh D Antigen Population Frequencies:
- Caucasians / White Population: $82\%$ to $85\%$ D-positive. Most probable Wiener phenotype: $R_1r$ .
- African Americans / Black Population: $92.5\%$ D-positive. Most probable Wiener phenotype: $R_0r$ or R_0R_0$.\n * Hispanic Population: 92.5\% D-positive.\n * Asian / East Asian Population: 98\% $to$ 99\% $D-positive ($ 99\% $D-positive in Chinese populations, with$ R_1R_1 being the predominant phenotype). D-negative blood is extremely rare in these regions.\n * Amish Population: Closed endogamous community exhibiting a significantly lower D-positive frequency of 60\% due to genetic isolation.\n\n* **Phenotype to Genotype Determinations**:\n * Serological testing determines the patient's *phenotype* (expressed antigens).\n * Exact *genotype* or zygosity (whether a D-positive individual is homozygous DD $or heterozygous$ Dd) cannot be determined by standard agglutination testing alone and requires specialized molecular DNA testing.\n * Because D$$ is inherited co-dominantly with other Rh alleles, any D-positive individual possesses at least two possible candidate genotypes based on population probability tables.