Blood Banking Fundamentals: Genetics, ABO/Rh, and Transfusion Safety

A set of practice flashcards covering the genetics of blood group antigens, ABO/Rh system, phenotype/genotype concepts, transfusion testing, and transfusion safety regulations.

Where do red cell antigens come from and how are they determined?

Antigens are encoded by DNA inherited from mom and dad; the genes on our chromosomes determine which antigens are expressed on the red cell membrane.

What is an allele and what is a locus?

An allele is a variant form of a gene; a locus is the specific location on a chromosome where the gene is found.

What do dominant, recessive, and codominant expression mean in genetics?

Dominant: expressed if present; recessive: expressed only with two copies; codominant: both alleles are expressed (e.g., A and B antigens together).

What is the difference between genotype and phenotype?

Genotype is the actual gene combination (e.g., AA, AO); phenotype is the observable trait (e.g., blood type A).

What is a Punnett square used for?

To predict possible offspring genotypes/phenotypes from parental genotypes.

What is an antithetical allele pair?

A pair of alleles at the same locus that are conceptually opposite (e.g., A and a); they can determine dominance relationships.

What is the ABO blood group system?

A system with A and B antigens on red cells and O as absence of both; A and B antigens are added to the H precursor by transferase enzymes encoded by ABO genes.

What is the H antigen and why is it essential for ABO expression?

H is the precursor on which A or B antigens are built; without H, A or B cannot be expressed (Bombay phenotype occurs when H is absent).

What is the secretor gene SE and what does it determine?

SE determines whether ABO antigens appear in secretions (secretors) or only on red cells (non-secretors); about 80% are secretors.

What are A1 and A2 subgroups and their clinical significance?

A1 is about 80% of A; A2 is ~20%; A2 individuals can have anti-A1 antibodies; subgroups like A3 can give mixed-field reactions.

What is the Bombay phenotype and its transfusion implications?

Bombay (Oh) lacks H antigen (hh); presents as O but has anti-H; can only receive Bombay blood and is extremely rare.

What is antigen dosage in ABO expression?

Dosage refers to two copies of the same gene (homozygous) producing stronger antigen expression; two different alleles (heterozygous) produce weaker expression.

Why is the Rh (D) antigen important in transfusion medicine?

D is highly immunogenic; its presence or absence defines Rh positive/negative and influences antibody formation and HDN risk.

Explain cis vs trans arrangement of Rh genes and its effect on D expression.

Cis: D is inherited on the same chromosome arm with C or E; trans: D is on the opposite arm. Cis usually yields stronger D expression; trans can weaken it.

What is forward typing versus antibody screening in blood banking?

Forward typing (phenotyping) detects antigens on patient red cells; antibody screening tests the patient plasma for alloantibodies against red cell antigens.

If one parent is AB and the other is O, what are the possible offspring blood types?

Possible offspring are A or B (types A or B); AB is not produced from AB x O.

If the parental genotypes are AA and BB, what are the offspring genotypes/phenotypes?

All offspring are AB (phenotype AB, genotype AB).

If one parent is AA and the other is BO, what is the probability of AB offspring?

50% AB offspring (the other 50% would be A from AA x BO).

What is the difference between type and screen and type and cross orders?

Type and screen orders determine blood type and screen for antibodies; type and cross orders reserve units and perform a crossmatch for compatibility.

Why is ABO compatibility critical for red cell transfusions?

To prevent transfusion reactions where donor red cells are destroyed by recipient antibodies, potentially leading to organ failure.

What are two independent identifiers used to label blood samples?

Two identifiers such as the patient’s name and date of birth (or MRN) to ensure correct matching.

What are the five elements that must be on a blood specimen label?

Two independent identifiers (name, DOB), collection date, collection time, collector (and collector ID).

What does WHIBBIT stand for in transfusion safety?

Wrong Blood In Tube; a label or sample associated error that can cause transfusion mistakes.

Why are pink-top tubes used in the blood bank, and how are they different from purple-top tubes?

Pink-top tubes are designated for blood bank samples to route testing correctly; purple-top tubes are for hematology; color-coding helps prevent misrouting.

What does QNS stand for and what does it mean?

Quantity Not Sufficient; not enough sample to perform testing.

What is EDTA and why is it used in blood banking samples?

EDTA is an anticoagulant that prevents clotting, preserving the sample for accurate testing.

What percentage of the population are secretors, and why does this matter?

About 80% are secretors; secretors express ABO antigens in secretions, which can aid certain types of testing and discrepancy resolution.

Why is the D antigen considered the most immunogenic antigen in blood banking?

Because exposure to D antigen most often elicits an anti-D antibody response, leading to transfusion reactions and HDN if not matched.

Which immunoglobulin class are ABO antibodies primarily, and which can cross the placenta?

ABO antibodies are predominantly IgM (large pentamers); IgG anti-A/anti-B can cross the placenta and affect fetal RBCs.

What are lectins and how are they used in ABO testing?

Lectins are plant-derived reagents used to identify specific ABO subtypes (e.g., anti-A1, anti-H) by binding particular antigens.

Why must the H antigen be present for expression of A and B antigens?

H is the backbone carbohydrate chain; A and B antigens are added onto H; without H, A and B cannot be formed.

What is the mixed-field reaction, and which ABO subgroups can cause it?

Mixed-field occurs when two populations (donor and recipient) react differently in the same tube; seen with A3 or B3 subgroups producing mixed-field results.

What is Landsberger’s rule in ABO testing?

If you have the antigen, you cannot form the corresponding antibody against it.