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Blood Grouping: Minor Blood Group Systems

Overview

• Hemolytic transfusion reactions (HTR) - both immediate and delayed
• Hemolytic disease of the fetus and newborn (HDFN)
• Difficulties in cross-matching (especially in multiply-transfused patients)

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Key Concept: Carbohydrate vs. Protein Antigens

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Relative Immunogenicity of Minor Blood Group Antigens

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The Major Minor Blood Group Systems

1. Rh System (Minor Antigens: C, c, E, e)

• Encoded by two closely linked genes on chromosome 1: RHD and RHCE
• The RHCE gene encodes 4 antigens: C, c, E, e
• All Rh antigens are protein antigens (non-glycosylated, hydrophobic transmembrane proteins)
• All Rh antibodies except anti-D show dosage (react stronger with homozygous cells)
• All Rh antigens are enhanced by enzymes (ficin, papain)
• Rh antibodies cause HTR and severe HDFN

• Stomatocytosis
• Chronic hemolysis
• Enhanced osmotic fragility
• Must only receive Rh-null blood

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2. Kell System

• K (Kell) antigen is the most immunogenic non-ABO, non-D antigen (10% sensitization rate)
• K antigen is present on a type II transmembrane glycoprotein (endothelin-converting enzyme family)
• Kell gene is on chromosome 7
• Related antigen k (Cellano) is present in 99.8% of people (high frequency antigen)
• The Kell antigen is associated with the McLeod phenotype: absent Kx antigen, reduced Kell antigens, acanthocytosis, and late-onset neuromuscular disease

• Kell antibodies are IgG, warm-reacting, acquired through transfusion or pregnancy
• Can cause severe HTR and severe HDFN (Kell HDFN is unusual - it suppresses fetal erythropoiesis in addition to hemolysis)
• Kell antigens are NOT enhanced (actually destroyed) by most proteolytic enzymes - unlike Rh

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3. Kidd System (Jk)

• Two antigens: Jka and Jkb, encoded on chromosome 18
• Kidd antigens are urea transporters (UTB)
• The Jk(a-b-) phenotype (null) is rare, seen in Finnish or Polynesian/Pacific Islander individuals
  • Jk-null cells are resistant to lysis by 2M urea - used as a confirmatory test

• Most common cause of delayed hemolytic transfusion reactions (DHTR)
• Notoriously hard to detect - antibody titers fall below detection over time
• Show dosage effect - may only react with homozygous cells
• React at AHG phase only
• Always check blood bank records before transfusion - even a historical Kidd antibody without current detectability is reason to give Kidd antigen-negative blood
• Rarely cause HDN (antigen is weakly expressed on fetal cells)

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4. Duffy System (Fy)

• Two main antigens: Fya and Fyb, encoded on chromosome 1
• Duffy antigens are expressed on the DARC protein (Duffy Antigen Receptor for Chemokines)
• DARC is a receptor for Plasmodium vivax merozoites

• Rare in whites, but present in ~68% of Black Africans
• Confers natural resistance to Plasmodium vivax malaria
• Most Fy(a-b-) Black individuals do NOT form anti-Fy antibodies (gene expression silenced in erythroid tissue only), while Fy(a-b-) whites DO form antibodies

• Warm-reacting IgG, acquired
• Show dosage effect
• Duffy antigens are destroyed by enzymes (useful diagnostic feature)
• Can cause HTR and severe HDFN

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5. MNS System

• Two linked gene loci: GYPA (glycophorin A) and GYPB (glycophorin B)
• M and N antigens reside on glycophorin A
• S, s, and U antigens reside on glycophorin B

• s and U are high-frequency antigens (present in >98% of population)
• Anti-U is found in rare U-negative individuals (mostly Black)

• Anti-M and anti-N are usually cold-reacting IgM, naturally occurring, and clinically insignificant
• Anti-S, anti-s, anti-U are warm IgG, acquired, and can cause HTR and HDFN
• MNS antigens are destroyed by proteolytic enzymes (ficin, papain)

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6. Lewis System (Le)

• Lewis antigens are carbohydrate antigens, not true red cell antigens - they are plasma antigens absorbed onto the RBC surface
• Produced by the FUT3 (Le gene) enzyme acting on type 1 precursor chains
• Two antigens: Lea and Leb

• During pregnancy, women can acquire the Le(a-b-) phenotype and form anti-Lea or anti-Leb
• Lewis antibodies cannot harm the baby because fetal cells do not express Lewis antigens
• Transfused red cells shed their Lewis antigens and acquire the Lewis phenotype of the recipient
• Lewis antibodies are quickly adsorbed by free serum Lewis antigens
• Lea is the CA 19-9 epitope (tumor marker in pancreatic cancer)
• Leb is a receptor for Helicobacter pylori and Norwalk virus
• Antibodies are nearly always IgM, clinically insignificant

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7. P1PK / GLOB System

• Three antigens: P1, P (now called GLOB), and Pk - but only P1 belongs to the official P1PK blood group system; P and Pk belong to the GLOB collection
• P antigen is the receptor for Parvovirus B19 (causes fifth disease / erythema infectiosum)
• P antigen is also the target of the Donath-Landsteiner antibody in paroxysmal cold hemoglobinuria (PCH)

• Individuals with P2 may make anti-P1: usually IgM, reactive at 4°C, clinically insignificant
  • Anti-P1 titers elevated in: echinococcal infection and bird handlers
  • Anti-P1 can be inhibited by hydatid cyst fluid and pigeon/turtledove egg whites
• Individuals with the rare p phenotype make potent anti-PP1Pk - associated with recurrent spontaneous abortions

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8. I/i System

• I and i antigens are epitopes within the ABH carbohydrate chains
• i antigen = unbranched oligosaccharides (predominant in neonates and cord blood)
• I antigen = branched oligosaccharides (increases with age; predominant in adults)

• Anti-I = autoantibody, usually clinically insignificant cold agglutinin
  • Associated with Mycoplasma pneumoniae infection (cold agglutinin disease)
• Anti-i = autoantibody, clinically insignificant
  • Associated with Epstein-Barr virus (EBV/Infectious mononucleosis) infection

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9. Lutheran System

• Two antigens: Lua and Lub
• Lub is high-frequency (present in 99% of population); Lua is in only ~7%
• ~93% are Lu(a-b+), ~7% are Lu(a+b+)
• Lutheran antigen expression is increased on sickle cells
• Lutheran antigenicity is destroyed by enzymes and by 2-ME and DTT (useful in antibody workup)
• Lutheran antibodies:
  • Usually anti-Lua specificity
  • Clinically insignificant cold-reacting IgM
  • Show mixed-field reactions (characteristic feature)

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Clinical Significance Summary Table

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Why Minor Blood Group Systems Matter Clinically

1. Sickle cell / thalassemia patients requiring chronic transfusions frequently develop alloantibodies to Rh (C, E), Kell, and Duffy antigens - these can make future cross-matching extremely difficult.
2. Delayed hemolytic transfusion reactions (DHTR) - Kidd is the most common cause. A patient previously sensitized re-develops antibodies on re-exposure, causing hemolysis 3-14 days post-transfusion.
3. HDFN - Kell, Rh(c, E), and Duffy are major causes of HDFN other than RhD.
4. Plasma vivax malaria resistance - The Duffy-null phenotype (common in West Africa) provides natural protection.
5. Infectious associations - Anti-I (Mycoplasma), Anti-i (EBV), P antigen (parvovirus B19 receptor), Leb (H. pylori receptor).

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Transfusion Medicine Basics - What Everyone in the Blood Bank Must Know

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1. Blood Component Characteristics and Storage

Red Blood Cells (pRBCs)

• Most common indication: symptomatic anemia, acute blood loss
• Transfusion threshold: generally Hb <7 g/dL (stable); <8 g/dL (cardiac disease or perioperative)

Platelets

• Indication: thrombocytopenia with bleeding, prophylactic threshold typically <10,000-20,000/µL
• Platelets have the highest risk of bacterial contamination (stored at room temp)

Fresh Frozen Plasma (FFP)

• Indications: coagulopathy with active bleeding, massive transfusion, TTP (plasmapheresis), warfarin reversal when Vitamin K too slow
• Group AB plasma = universal donor (no anti-A or anti-B)
• Not for volume expansion alone

Cryoprecipitate (Cryo)

• Primary indication: fibrinogen replacement (fibrinogen <100-150 mg/dL with bleeding)
• Also used in: hemophilia A (when factor concentrate unavailable), vWD (type 1/2A), Factor XIII deficiency, uremic bleeding (topical fibrin)
• Dose: 1 unit per 7-10 kg body weight (adult: ~10 units)

Granulocytes

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2. Pretransfusion Testing: Type, Screen, and Crossmatch

Step-by-Step Workflow

• Label at the bedside with 2 unique patient identifiers
• Record date/time and phlebotomist identity
• Any discrepancy = discard and recollect
• If patient transfused or pregnant in the past 3 months: specimen valid only 3 days
• Stable patients with no recent pregnancy/transfusion: specimen valid up to 1 month
• Retain pre- and post-transfusion specimens for 7 days

• Forward (cell) typing: patient's red cells tested with anti-A (blue) and anti-B (yellow) reagents
• Reverse (serum) typing: patient's serum tested against known A1 and B reagent cells
• These must match - discrepancy = investigate before issuing any blood (only O can go out pending resolution)
• Recipient does NOT need to be tested for weak D (only donors are tested for weak D)

Common ABO Forward/Reverse Discrepancies

• Patient serum screened against O panel cells containing all clinically significant antigens
• Performed in LISS or PEG, at 37°C and AHG phase
• Positive screen = identify the antibody before crossmatching

Key rule: If a patient has a historically documented alloantibody (even if currently undetectable), always provide antigen-negative blood for that specificity.

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3. ABO/D Compatibility Rules

For Red Blood Cells (must be MAJOR ABO compatible)

For Plasma/FFP (must be MINOR ABO compatible - no ABA/anti-B in plasma attacking patient RBCs)

• Group AB plasma = universal plasma donor
• Group A plasma is increasingly used for massive transfusion (only 4% of people are AB)

For Rh (D antigen)

• D-negative females of childbearing potential: must receive D-negative RBCs (prevents anti-D that causes HDFN in future pregnancies)
• D-negative males and post-menopausal females: can receive D-positive in shortage situations
• Risk of anti-D sensitization from 1 D-positive unit: ~10-20%
• Plasma and FFP: not D-matched (no RBCs in product)

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4. Blood Component Modifications

Leukoreduction

• Removes WBCs to <5 × 10⁶ per unit
• Indications / Benefits:
  • Reduces febrile nonhemolytic transfusion reactions (FNHTRs)
  • Reduces CMV transmission (CMV-seronegative equivalent)
  • Reduces HLA alloimmunization (platelet refractoriness)
  • Reduces immunomodulation
• Given to: chronically transfused patients, transplant candidates, premature neonates

Irradiation

• Gamma irradiation (25 Gy) or X-ray irradiation kills donor T lymphocytes
• Purpose: Prevents transfusion-associated graft-versus-host disease (TA-GvHD)
• Indications:
  • Immunocompromised recipients (hematologic malignancies, HSCT, congenital immunodeficiency)
  • Directed donations (from relatives - HLA similar)
  • Granulocyte transfusions (always)
  • Intrauterine transfusions
• Irradiation shortens RBC shelf life - must use within 28 days of irradiation or original expiry (whichever is sooner)

Washing

• RBCs/platelets washed with saline to remove plasma proteins
• Indications: IgA deficiency (anaphylaxis risk), severe recurrent allergic reactions, hyperkalemia (neonates)
• Washed RBCs: shelf life reduced to 24 hours after washing

Cytomegalovirus (CMV)-Negative Products

• From CMV-seronegative donors
• Leukoreduced products are considered CMV-safe equivalent (leukocytes carry CMV)
• Indicated for: seronegative immunocompromised patients, seronegative pregnant women, neonates

Pathogen Reduction

• Treats platelets/plasma with chemicals (amotosalen + UV light, riboflavin + UV light) to inactivate pathogens
• AABB requires methods to detect/prevent bacterial contamination in platelets

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5. Direct and Indirect Antiglobulin Tests (DAT/IAT)

Indirect Antiglobulin Test (IAT / Indirect Coombs)

• Detects antibodies in patient SERUM against foreign RBC antigens
• Used in: antibody screen, crossmatch, prenatal testing
• Positive = patient has alloantibody in circulation

Direct Antiglobulin Test (DAT / Direct Coombs)

• Detects antibodies already COATING patient's RBCs in vivo
• Uses polyspecific AHG (detects both IgG and C3d on cells)
• Positive = RBCs coated with IgG or complement in vivo

Drug-Induced Positive DAT - Key Drugs to Remember

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6. Transfusion Reactions - Recognition and Management

Risk of Transfusion Complications (approximate rates)

Transfusion Reaction Workup (do this for every suspected reaction)

1. Stop the transfusion - disconnect but keep IV access
2. Clerical check - verify patient ID vs blood unit label (most common cause of severe hemolytic reactions is clerical error)
3. Visual inspection - check patient plasma/serum for pink/red hemolysis; urine dipstick for hemoglobinuria
4. Repeat ABO typing - on a post-transfusion sample
5. DAT - compare with pre-transfusion sample; DAT is positive in hemolytic reactions (but may be negative if all transfused cells already destroyed)
6. Notify blood bank physician - for review and further workup
7. Report to FDA and collecting facility for fatalities or serious morbidity within 7 days

Note: If blood drawn >8 hours post-event may appear visually clear but serum bilirubin will remain elevated for 24-36 hours. Hemoglobinuria (dipstick+) without hematuria (microscopy-) = intravascular hemolysis confirmed.

Individual Reactions at a Glance

• Most common reaction (~1 in 200)
• Temp rise ≥1°C with no other explanation
• Cause: cytokines from WBCs in stored blood
• Treatment: antipyretics; continue with leukoreduced products in future
• Prevention: prestorage leukoreduction

• Second most common (~1 in 100-300)
• Urticaria → anaphylaxis spectrum
• Cause: IgE-mediated reaction to plasma proteins; anti-IgA in IgA-deficient patients
• Mild: antihistamines; may restart transfusion after symptom resolution
• Anaphylaxis: epinephrine (parenteral), stop transfusion permanently
• IgA-deficient patients with anaphylaxis: must receive IgA-deficient or washed units

• Usually ABO incompatible blood (almost always a clerical/labeling error)
• Intravascular hemolysis via complement activation
• Symptoms: fever, chills, flank/back pain, hemoglobinuria (red/cola urine), hypotension, DIC, renal failure
• Treatment: stop transfusion, aggressive IV fluids, maintain urine output, treat DIC, dialysis if needed

• Most common cause: Kidd (Jk) antibodies - anamnestic response
• Occurs 3-14 days post-transfusion
• Extravascular hemolysis, falling Hb, new positive DAT, new alloantibody identified
• Usually less severe than AHTR; manage supportively

• Currently the most common cause of transfusion-related fatality
• Non-cardiogenic pulmonary edema within 6 hours of transfusion
• Mechanism: donor anti-HLA class I/II or antigranulocyte antibodies activate recipient neutrophils
• Clinical: acute hypoxia, bilateral lung infiltrates, no evidence of fluid overload (PCWP <18)
• Treatment: supportive (oxygen, ventilation); self-limiting within 48-96 hours
• Prevention: use male plasma donors or female donors screened for HLA antibodies
• Mortality: 5-10%

• Cardiogenic pulmonary edema from volume overload
• At-risk: elderly, cardiac patients, infants, renal failure
• Distinguish from TRALI by: BNP elevated, responds to diuretics, PCWP >18
• Treatment: diuretics (furosemide), slow transfusion rate

• Donor T lymphocytes engraft in immunocompromised recipient and attack host tissue
• Presents 8-10 days post-transfusion: dermatitis (periauricular, palmar, plantar) + enterocolitis (watery diarrhea) + hepatitis + pancytopenia
• >90% fatal; no effective treatment
• Prevention: irradiate cellular components for at-risk patients

• Mostly with platelets (room temperature storage → bacterial growth)
• High fever, rigors, hypotension, rapid deterioration
• Treatment: stop transfusion, cultures (patient + unit), broad-spectrum antibiotics, hemodynamic support
• Prevention: bacterial screening of platelets, first 15-30 mL discarded during donation (diversion pouch)

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7. The Transfusion Procedure Itself - Practical Rules

• Must have physician order and written informed consent
• Confirm patient identity at bedside with 2 unique identifiers before hanging
• Use standard blood infusion set with 170 µm filter
  • Same filter can be used for up to 4 hours (multiple units)
  • NOT a leukoreduction filter
• Only co-infuse with 0.9% Normal Saline - no dextrose (causes hemolysis), no lactated Ringers (causes clotting), no medications
• Transfusion must start within 30 minutes of issue from blood bank
• Must complete within 4 hours of starting
• Observe patient for the first 15 minutes at a slow rate (~2 mL/min) - most acute reactions begin here
• Record vitals before, during (15 min), and after transfusion

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8. Massive Transfusion Protocol (MTP)

• Balanced resuscitation ratio: 1:1:1 (pRBC : FFP : Platelets) - mirrors whole blood
• Goal: replace factors, prevent dilutional coagulopathy and thrombocytopenia
• Use Group O pRBCs and Group AB (or A) plasma before type is known
• Monitor: Ca²⁺ (citrate in stored blood chelates calcium → give calcium with every 4 units), temperature (hypothermia worsens coagulopathy), pH
• Cryoprecipitate when fibrinogen <150 mg/dL with ongoing bleeding
• Switch to type-specific components as soon as ABO type is confirmed

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Quick Reference Summary Card

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