Study guide
8.2 Duffy System
Scientist in Blood Banking, BB ASCP
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1 Introduction & Exam Overview
1.1 Current Exam Facts 1.2 Eligibility Routes and Application 1.3 CMP, Credential Citation, and BB vs SBB
2 Donors and Collection
2.1 Donor Qualification 2.2 Collection Methods and Adverse Reactions 2.3 Special Donations
3 Processing, Storage, and Component QC
3.1 Processing: FDA/AABB Requirements, Testing, and Labeling 3.2 Anticoagulants, Additives, and Storage Temperatures 3.3 Transportation, Stored-Product Properties, and Expiration 3.4 Blood Component Quality Control
4 Blood Components
4.1 Red Blood Cells, Whole Blood, Washed, Frozen, and Rejuvenated 4.2 Platelets, Plasma, and Cryoprecipitated AHF 4.3 Granulocytes, Leukoreduced, and Irradiated Components 4.4 Apheresis Products, Fractionation, and Hematopoietic Progenitors
5 Blood Group Genetics and Transfusion Role
5.1 Basic and Molecular Genetics 5.2 Inheritance of Blood Groups 5.3 Immunogenicity and Antigen Prevalence
6 ABO and Lewis Systems
6.1 ABO Biochemistry and Antigens 6.2 ABO Antibodies, Discrepancies, and Subgroups 6.3 Lewis System
7 Rh System
7.1 Rh Genetics and Antigens 7.2 Weak D, Partial D, and Rh Antibodies 7.3 Rh Phenotyping and Clinical Significance
8 Kell, Duffy, Kidd, and MNS
8.1 Kell System 8.2 Duffy System 8.3 Kidd System 8.4 MNS System
9 Additional Systems and Antigens
9.1 P1PK, Globoside, and Ii Systems 9.2 Lutheran and Other Blood Group Systems 9.3 High- and Low-Prevalence Antigens 9.4 HLA, Platelet-Specific, and Granulocyte-Specific Antigens
10 Immunology Foundations
10.1 Immune Response, B and T Cells, Macrophages 10.2 Immunoglobulins: Classes, Structure, and Properties 10.3 Antigen-Antibody Interactions and Testing 10.4 Complement Pathways and Biologic Properties
11 Physiology, Hemostasis, Cytopenias, and Transplantation
11.1 Physiology of Blood 11.2 Hemostasis and Coagulation 11.3 Immune and Nonimmune Cytopenias 11.4 Solid Organ and HPC Transplantation
12 Hemolytic Disease of the Fetus and Newborn
12.1 HDFN Pathophysiology 12.2 HDFN Detection 12.3 HDFN Treatment and Prevention
13 Routine Serologic Testing
13.1 Blood Grouping Tests 13.2 Antibody Detection 13.3 Compatibility Testing and Crossmatch 13.4 Direct Antiglobulin Testing
14 Antibody Identification
14.1 Antibody Identification and Rule-Out 14.2 Clinical Significance, Dosage, and Multiple Antibodies 14.3 Panel Problem-Solving and Common Traps
15 Reagents and Special Techniques
15.1 Antiglobulin Sera, Grouping Sera, and Reagent Red Cells 15.2 Enzymes, Enhancement Media, and Lectins 15.3 Adsorption, Elution, Titration, and Cell Separations 15.4 Molecular, Solid-Phase, Column, ELISA, and Related Methods
16 Serologic QA and Special Reagents
16.1 Blood Sample Quality Assurance 16.2 Reagent and Procedure Quality Assurance 16.3 Thiol Reagents, Neutralization, Chloroquine, EGA, and Immunofluorescence
17 Indications and Component Therapy
17.1 Indications for Transfusion 17.2 Red Cell and Whole Blood Therapy 17.3 Platelet, Plasma, and Cryoprecipitate Therapy 17.4 Special Component Therapy
18 Adverse Effects of Transfusion
18.1 Immunologic Transfusion Reactions 18.2 Nonimmunologic Transfusion Reactions 18.3 Transfusion-Transmitted Diseases
19 Apheresis, Administration, and Patient Blood Management
19.1 Apheresis and Extracorporeal Circulation 19.2 Blood Administration 19.3 Patient Blood Management
20 Laboratory Operations
20.1 Quality Assessment and Troubleshooting 20.2 Quality Control, Proficiency Testing, and Regulation 20.3 Laboratory Safety 20.4 Laboratory Mathematics and Instrumentation
8.2 Duffy System
Key Takeaways Explain This Topic AI
Fya and Fyb are antithetical ACKR1 (FY) antigens; Fy3 is present on all red cells except true Duffy-nulls.
The common African Fy(a−b−) type is an FY*B GATA-1 promoter mutation that silences Duffy on red cells only; tissues still express Fyb and P. vivax invasion is reduced.
Anti-Fya and anti-Fyb are IgG, clinically significant, show dosage, and are destroyed by ficin and papain.
Anti-Fy3 is made by true Duffy-nulls and still reacts with enzyme-treated Fy(a+) or Fy(b+) cells.
Do not prophylactically issue Fyb− units to Fy(a−b−) African-American patients who have no antibody — they usually have tissue Fyb and do not make anti-Fyb.
Last updated: August 2026
8.2 Duffy System
Quick Answer: Fya and Fyb are antithetical antigens on ACKR1 (DARC, FY) . Fy3 is a high-prevalence Duffy antigen present whenever any Duffy protein is on the red cell. The common Fy(a−b−) type in African ancestry is an FY B GATA-1 promoter mutation: red cells lack Duffy , but tissues still express Fyb , and the phenotype confers Plasmodium vivax resistance. Anti-Fya and anti-Fyb are IgG , clinically significant, show dosage , and are destroyed by ficin and papain . Anti-Fy3 is made by true nulls and still reacts with enzyme-treated cells . Do not give Fy(a−b−) African-American patients Fyb− units just because they type Fy(b−) unless the antibody is present.
Duffy is II.B.9 on the June 9, 2026 outline because candidates merge three different facts into one wrong action: the red cell types Fy(b−), so they issue Fyb− blood, so they assume the patient is a Duffy-null who will make anti-Fy3. Most African-ancestry Fy(a−b−) patients are not true nulls.
Fya, Fyb, and Fy3 on ACKR1
ACKR1 (atypical chemokine receptor 1), historically DARC or FY , lives on chromosome 1 — same chromosome as RH, different locus. The protein is a chemokine receptor that also happens to be the P. vivax / P. knowlesi red-cell invasion receptor. Fya (FY1) and Fyb (FY2) are antithetical and differ by a single amino acid ( Gly42Asp , the 125G>A SNP). Fy3 is an epitope on the Duffy protein that is present on Fy(a+) and Fy(b+) cells and absent only when the protein itself is missing from the membrane.
Classic teaching phenotypes:
- Phenotype European ancestry African ancestry What the red cell carries
- Fy(a+b−) ~17% ~9% Fya and Fy3
- Fy(a+b+) ~49% ~1% Fya, Fyb, and Fy3
- Fy(a−b+) ~34% ~22% Fyb and Fy3
- Fy(a−b−) Rare ~68% No Fya, Fyb, or Fy3 on the RBC
Fya is more immunogenic than Fyb , so anti-Fya is the Duffy antibody you identify most often. Both antibodies are IgG , 37 °C / AHG , clinically significant , and capable of HTR and HDFN . They show dosage : a heterozygous Fy(a+b+) cell can look weak or negative while a Fy(a+b−) cell is 2+ or 3+. Always include a double-dose (homozygous) cell before you rule Duffy out.
Ficin and papain destroy Fya and Fyb. A panel that goes silent after enzyme treatment is consistent with Duffy or MNS (M/N, sometimes S/s), not with Kidd, Rh, or Kell. That enzyme pattern is half of the identification. The other half is the GATA story below.
The GATA-1 promoter mutation is not a deleted gene
The common African Fy(a−b−) red-cell type is FY B with a GATA-1 binding-site mutation in the erythroid promoter (classically taught as −67T>C ). GATA-1 cannot dock, so ACKR1 is not transcribed in the red-cell lineage . Endothelium and other tissues use a different promoter, so Fyb is still made in tissues . A microarray that reads only the 125G>A Fya/Fyb SNP will call the person Fyb+ while the red cells type Fy(b−) . That genotype–phenotype mismatch was already the teaching example in Section 5.1. It is not a broken chip.
Three clinical payoffs sit on that single mutation:
Malaria. Plasmodium vivax (and P. knowlesi ) needs Duffy to invade the red cell. Fy(a−b−) red cells are resistant to P. vivax . This is not the main P. falciparum receptor story — do not swap the species on a BB item.
Anti-Fyb is unusual. The patient still sees Fyb in tissues, so the immune system is not looking at a foreign antigen. Anti-Fya can still form if the person is exposed to Fya+ red cells, because they never made Fya. Anti-Fy3 is also unusual in GATA-silenced patients, because they are not missing Duffy protein everywhere.
Do not prophylactically phenotype-match Fyb. A Fy(a−b−) African-American patient with a negative antibody screen does not automatically need Fy(a−b−) or Fyb− units. Issue units that match any identified antibody (often anti-Fya, or an unrelated antibody). Honoring a Fy(b−) type that is only a red-cell silence wastes antigen-negative inventory.
True Duffy-nulls are different. Rare inactivating ACKR1 mutations abolish the protein in all tissues . Those people can make anti-Fy3 (and sometimes anti-Fya, anti-Fyb, or anti-Fy5). Anti-Fy3 reacts with all cells except Fy(a−b−) and, critically, still reacts after ficin or papain because the Fy3 epitope is enzyme-resistant . That is how you separate anti-Fy3 from anti-Fya or anti-Fyb on an enzyme panel: Fya/Fyb disappear; Fy3 does not.
Fy^x is a weak Fyb that some reagents miss. The patient may type Fy(b−) with one clone and Fy(b+w) with another. Molecular typing or a different anti-Fyb clone prevents you from calling every weak Fyb a GATA silence.
Antibody identification and unit selection
Work a suspected Duffy antibody the way the bench actually works it:
Reactivity at AHG , stronger with Fy(a+b−) or Fy(a−b+) double-dose cells than with Fy(a+b+) cells ( dosage ).
Destroyed by ficin/papain if the specificity is anti-Fya or anti-Fyb.
Not destroyed by DTT — Duffy is not Kell.
If the serum still reacts with enzyme-treated Fy(a+) or Fy(b+) cells and is silent only with Fy(a−b−) cells, stop calling it anti-Fya and start calling it anti-Fy3 .
Once identified, honor anti-Fya and anti-Fyb for every red-cell transfusion. They are not Lewis. They are not room-temperature anti-M.
What you should not do is look at an African-American type-and-screen that reads Fy(a−b−), invent anti-Fyb that is not there, and deplete the Fy(b−) stock. Section 5.3 already ranked immunogenicity; this section ranks what the patient can actually make . GATA-silenced FY*B patients make anti-Fya far more often than anti-Fyb. Wait for the antibody.
If a chronically transfused sickle-cell patient of African ancestry needs extended matching, match C, E, K , and other demonstrated antibodies first. Adding prophylactic Fyb matching for a GATA Fy(a−b−) type is not the default BB rule unless a local SCD protocol says otherwise and the antibody history supports it.
Exam traps
Treating every Fy(a−b−) African-American patient as a true null who will make anti-Fy3.
Issuing Fyb− units solely because the red cells type Fy(b−).
Naming P. falciparum as the Duffy-receptor malaria. The BB pairing is P. vivax .
Expecting enzymes to destroy anti-Fy3. Fy3 is enzyme-resistant ; Fya and Fyb are not.
Ruling out anti-Fya on a single Fy(a+b+) heterozygous cell. Dosage will fool you.
Using DTT to strip Duffy. DTT is the Kell reagent, not the Duffy reagent.
Believing a Fyb+ microarray result contradicts Fy(a−b−) red cells — that is the GATA-1 signature, not a labeling error.