Blood Group Systems – Comprehensive Study Notes

Structural Classification of RBC Antigens

• Three broad membrane-anchor types recognised across systems
• Single-pass trans-membrane proteins (e.g., Kell, Diego, Cartwright/Yt, etc.)
• Multi-pass trans-membrane proteins (e.g., Kidd, Rh, RhAG, Duffy, MNS glycophorins)
• GPI-linked proteins (e.g., Cromer, Knops, Lutheran, JMH, LW)
• Carbohydrate antigens (e.g., ABO, H, P1, P, Pk, Lewis) project from glycan chains rather than peptide backbones
• Topological orientation
• NH_2 terminus generally extracellular; COOH terminus cytoplasmic
• Antigen-defining amino-acid changes usually lie in extracellular loops

Kell System (ISBT 006)

• Carrier: Kell glycoprotein – single-pass, 732-aa zinc endopeptidase (type II orientation)
• Associated XK protein forms Kell–Kx complex; absence → McLeod phenotype
• Clinically important antigens
• K (K1) vs k (K2): Met \rightarrow Thr at aa 193
• Kp^a/Kp^b/Kp^c: variants around aa 281 (Pro → Arg/Trp)
• Js^a/Js^b: Leu → Pro at aa 597
• Serology
• Antibodies are exposure-requiring, warm \text{IgG}
• Anti-K: second only to anti-D in immunogenicity; causes severe HTR & HDFN
• Anti-k rarer but of equal severity
• Transfusion rules
• Women of child-bearing potential must receive K-negative units
• Antigen-negative units cross-matched by IAT 37\,^{\circ}\mathrm{C}

Kidd System (ISBT 009)

• Carrier: urea transporter (SLC14A1); 10-pass membrane protein
• Molecular bases
• Jk^a/Jk^b: Asp 280 \leftrightarrow Asn (cDNA 838\;A\leftrightarrow G)
• Null phenotype Jk(a-b-) by silencing mutations or JK^{\text{XN}} promoter defect
• Serology
• Antibodies often mixed \text{IgG}+\text{IgM}, warm reactive
• Marked dosage – stronger with homozygous ("double dose") cells
• Exhibit antibody evanescence → titres may fall below detection
• Cause severe intravascular or delayed HTR; mild HDFN
• Laboratory pearls
• Include double-dose Jk(a+b-) & Jk(a-b+) cells in screening panels
• If anti-Jk detectable, supply Jk(a-b-) units ± rare donor file

Duffy System (ISBT 008)

• Carrier: DARC (ACKR1) – 7-pass chemokine receptor
• Fya/Fyb defined by Gly 42 \leftrightarrow Asp
• Fy^x phenotype: Arg89Cys (weak Fyb)
• Fy(b-) in Africans often Fy*B^{\text{ES}} (promoter silencer) – confers Plasmodium\,vivax resistance
• Serology
• Exposure-requiring warm \text{IgG}
• Marked dosage; titres may vary with time
• Anti-Fya/Fyb → delayed HTR, moderate HDFN
• Transfusion – give antigen-negative, IAT-compatible units

MNS System (ISBT 002)

• Carriers: Glycophorin A (M, N), Glycophorin B (S, s, U) – single-pass but multiple copies
• Antibody characteristics
• Anti-M, anti-N: naturally occurring \text{IgM}, cold reactive, rarely clinically significant (occasionally active at 37^{\circ}\mathrm{C})
• Anti-S, -s, -U: warm \text{IgG}, clinically significant (HTR, HDFN)

Lewis System (ISBT 007)

• Glycolipid antigens Lea/Leb adsorbed onto RBC from plasma; phenotype depends on FUT2 & FUT3 secretor genes
• Antibodies (mostly in Le(a-b-) individuals) are cold \text{IgM}, insignificant; rarely cause in-vitro incompatibility

P1PK & GLOB Systems (ISBT 003 & 028)

• Biochemical series of glycosphingolipids
• P1PK: P1, Pk (Gb3)
• GLOB: P (Gb4; “globoside”)
• Antibodies
• Usually cold \text{IgM} anti-P1/anti-Pk/anti-P – interfere with reverse grouping if tests not at 37^{\circ}\mathrm{C}
• Rare warm anti-P1 reported; usually not significant
• Transfusion – un-phenotyped units acceptable if IAT cross-match compatible

Lutheran System (ISBT 005)

• Antigens: Lu^a (low-freq), Lu^b (high-freq); single-pass adhesion receptor
• Anti-Lu^a: cold \text{IgM} → benign
• Anti-Lu^b: warm \text{IgG} → may cause mild DHTR but seldom HDFN

Diego System (ISBT 010)

• Carrier: Band-3 (anion exchanger 1)
• Dia (low) / Dib (high) – significant antibodies cause DHTR & severe HDFN
• Important in Indigenous American & East Asian populations

Antibody Detection & Screening Panels

• Reagent requirement (UK BSH/ANZSBT standard) – each O screening cell set must express
• C,\;c,\;D,\;E,\;e,\;M,\;N,\;S,\;s,\;K,\;k,\;Fya,\;Fyb,\;Jka,\;Jkb,\;Le^a,\;Le^b
• Double-dose phenotypes for Kidd & Duffy plus SS/ss desirable
• Interpretation table (excerpt)
• Clinically significant antibodies (anti-K, ‑Fya, ‑Jkb, ‑S, etc.) → select antigen-negative, IAT 37^{\circ}\mathrm{C} compatible units
• Rarely significant antibodies (anti-M active only below 37^{\circ}\mathrm{C}, HTLA) → follow local or reference-lab advice

Dosage Phenomenon (Heterozygosity Effect)

• Heterozygous expression gives fewer antigen sites per RBC → weaker reaction
• Example (Kidd)
• Genotype Jk^{a}Jk^{a} (double dose) → 3+ with anti-Jka
• Genotype Jk^{a}Jk^{b} (single dose) → 1+ reaction
• Ensures screening panels include homozygous cells to avoid false negatives

Antibody Titre & Evanescence Concept

• Initial allo-immunisation raises titre above laboratory detection limit
• Without further exposure, memory B-cells wane → antibody becomes undetectable (“evanescence”)
• Re-exposure triggers brisk anamnestic rise → delayed HTR diagrammatically:
\text{1st exposure} \;\xrightarrow{time}\; \downarrow\text{titre below detection} \;\xrightarrow{2nd exposure}\; \uparrow\uparrow \text{pathogenic level}

Summary of Clinical Significance (Quick Reference)

• Severe HTR & HDFN: Anti-D, ‑K, ‑k, ‑S/s/U, ‑Fya/Fyb, ‑Jka/Jkb, ‑Dia/Dib
• Severe HTR only: Anti-Jka/Jkb (intravascular), occasional anti-P complement-binding
• Mild/rarely significant: Anti-M, ‑N (unless 37^{\circ}\mathrm{C} reactive), Lewis, most P1PK

Practical Transfusion Rules

• Always match for ABO/RhD; additionally match K for females of child-bearing potential
• If clinically significant antibody present or history positive, issue antigen-negative, IAT 37^{\circ}\mathrm{C} cross-match-compatible units
• Warm autoantibodies or multiple alloantibodies – seek reference laboratory assistance

Ethical & Public-Health Notes

• Selective antigen matching minimises allo-immunisation burden, reducing future transfusion difficulty
• Ethnic diversity in antigen prevalence (e.g., Fy(b-) in Africans, high Dia in South American natives) necessitates diverse donor pools