ABO Blood Group System — antigens, antibodies, genetics and haemolytic risk

Definition / Overview

Pre-transfusion compatibility testing encompasses a sequence of laboratory procedures designed to ensure that donor red cells are serologically compatible with the recipient and that clinically significant alloantibodies are detected before transfusion. The core components are:

• ABO and RhD grouping of both patient and donor
• Antibody screening of patient plasma/serum against a panel of reagent red cells
• Antibody identification when screening is positive
• Red cell phenotyping of patient and/or donor
• Crossmatching (serological or electronic) to confirm compatibility

Errors in this process, particularly ABO-incompatible transfusion, remain a leading cause of transfusion-related mortality. Robust technique, automation, and quality systems are therefore central to safe practice.

ABO and RhD Grouping

Principles

ABO grouping relies on two complementary reactions:

• Forward (cell) group: patient red cells tested against monoclonal anti-A and anti-B reagents
• Reverse (serum/plasma) group: patient plasma tested against known A1 and B reagent red cells, exploiting the predictable presence of naturally occurring ABO antibodies (Landsteiner's rule)

Concordance between forward and reverse groups is mandatory. Discrepancies require investigation before blood is issued.

RhD typing uses at least two IgM monoclonal anti-D reagents. Weak D variants (previously termed Du) may require indirect antiglobulin test (IAT) confirmation. Patients with weak D or partial D phenotypes are managed as RhD-negative for transfusion purposes to avoid alloimmunisation.

ABO Grouping Results

Reagents

IgM monoclonal reagents are standard for ABO, RhD, and K typing. They directly agglutinate red cells without requiring anti-human globulin (AHG). Polyclonal antisera carry a higher risk of contaminating antibodies and require extensive validation before use.

Sample Requirements and Safety

• A second independently collected sample is required before issuing blood to a patient with no historical group on record, to guard against "wrong blood in tube" errors
• Barcoded sample labelling and electronic transfer of results reduce transcription errors
• In the UK, approximately 97% of routine group-and-screen testing is performed on fully automated platforms

Antibody Screening

Purpose and Principles

Antibody screening detects clinically significant unexpected red cell alloantibodies in patient plasma before transfusion. It uses a panel of two or three group O reagent red cells selected to carry, between them, the following antigens in at least single dose (homozygous expression preferred for Fy$^a$ and Jk$^a$):

D, C, E, c, e, M, N, S, s, P1, Le$^a$, Le$^b$, K, k, Fy$^a$, Fy$^b$, Jk$^a$, Jk$^b$

Ideally one cell is R1R1 (DCe/DCe) and one is R2R2 (DcE/DcE) to ensure all major Rh antigens are represented.

Technique

The IAT (indirect antiglobulin test) at 37°C is the standard method. Low ionic strength solution (LISS) is used to enhance antibody uptake and reduce incubation time. Saline tests at room temperature are not required for routine screening; antibodies reactive only at temperatures below 37°C are generally not clinically significant.

Procedure outline:

1. Mix patient plasma with reagent red cells in LISS
2. Incubate at 37°C for 15-20 minutes
3. Wash cells to remove unbound immunoglobulin
4. Add anti-human globulin (AHG) reagent
5. Centrifuge and read for agglutination
6. Add IgG-sensitised check cells to all negative tests to confirm AHG was active

Technology Platforms

CAT systems available include gel matrix cards (Bio-Rad ID cards, Grifols) and glass microbead matrix (Ortho BioVue). Solid-phase systems include Immucor Capture-R and Bio-Rad Solidscreen II.

Controls

• A weak anti-D control should be run regularly to confirm the sensitivity of the entire procedure
• Check cells (IgG-sensitised red cells) must be added to every negative IAT result; failure to agglutinate invalidates the test
• Anti-S can serve as a post-cleaning control on automated analysers, as the S antigen is highly sensitive to bleach (hypochlorite) residue

Antibody Identification

When Required

Any positive antibody screen requires full identification before antigen-negative blood is selected for transfusion.

Panel Testing

An identification panel of 8-12 fully phenotyped group O red cells is tested against the patient's plasma using:

1. IAT (primary technique)
2. A second sensitive technique such as enzyme-treated cells, polyethylene glycol (PEG), or manual polybrene

Enzyme treatment (papain or bromelin) enhances detection of IgG antibodies to Rh, Kidd, and Lewis antigens but destroys M, N, S, Fy$^b$ antigens. This property is diagnostically useful: an antibody reactive with enzyme-treated cells but not with untreated cells is unlikely to be anti-M, -N, -S, or -Fy$^b$.

Interpretation

• A positive reaction pattern is compared against the panel's antigen profile to identify which antigen(s) correlate with reactivity
• Rule-in: the antibody reacts with all cells carrying the relevant antigen
• Rule-out: the antibody does not react with cells lacking the antigen
• At least three antigen-positive and three antigen-negative cells should support the specificity before it is accepted
• Multiple antibodies require careful dissection, often using adsorption/elution techniques

Clinically Significant Antibodies

Autoantibodies

Warm autoantibodies (IgG, reactive at 37°C) can mask underlying alloantibodies. Adsorption techniques using autologous or allogeneic red cells are required to remove the autoantibody before alloantibody identification. Cold autoantibodies (IgM, reactive below 30°C) are usually clinically insignificant unless the thermal amplitude extends to 37°C.

Red Cell Phenotyping

Indications

• Patients with known or suspected alloantibodies (to select antigen-negative units)
• Patients likely to require long-term transfusion support (sickle cell disease, thalassaemia, myelodysplastic syndrome) to establish an extended phenotype before alloimmunisation
• Patients with autoimmune haemolytic anaemia (to guide adsorption studies)
• Antenatal patients with clinically significant antibodies

Antigens Routinely Phenotyped

Beyond ABO and RhD, extended phenotyping typically includes:

• Rh: C, c, E, e (and where relevant, Cw, V, VS)
• Kell: K, k, Kp$^a$, Kp$^b$, Js$^a$, Js$^b$
• Duffy: Fy$^a$, Fy$^b$
• Kidd: Jk$^a$, Jk$^b$
• MNS: M, N, S, s
• Lewis: Le$^a$, Le$^b$

Molecular Genotyping

Serological phenotyping is unreliable when:

• The patient has been recently transfused (circulating donor cells)
• Autoantibodies interfere with typing reagents
• Reagents for rare antigens are unavailable

In these situations, molecular blood group genotyping (PCR-based or microarray) predicts phenotype from DNA. Most blood group polymorphisms arise from single nucleotide polymorphisms (SNPs). Key applications include:

• Fetal RhD genotyping from cell-free fetal DNA in maternal plasma (from approximately 11 weeks gestation) to guide anti-D prophylaxis in RhD-negative mothers
• Extended genotyping of sickle cell and thalassaemia patients before chronic transfusion programmes
• Resolving discrepant serology

Crossmatching

Methods

Electronic issue is safe and efficient for the majority of elective transfusions and reduces laboratory workload without compromising safety, provided the antibody screen is negative and two historical concordant groups exist.

Emergency Blood Issue

When blood is required before full compatibility testing:

1. Immediate: group O RhD-negative red cells (uncrossmatched)
2. Within 10-15 minutes: ABO/RhD-compatible blood after rapid grouping
3. Within 30-45 minutes: fully crossmatched blood

RhD-negative O blood should be reserved for females of childbearing potential and neonates; RhD-positive O blood is acceptable for males and post-menopausal females in extremis.

Quality Assurance and Automation

Internal Quality Control

• Positive and negative controls must be included with every batch of tests
• Check cells validate every negative IAT result
• Weak anti-D controls confirm procedural sensitivity
• Reagent lot changes require parallel testing before implementation

External Quality Assurance (EQA)

Participation in national EQA schemes (e.g. RCPA QAP, UKNEQAS Blood Transfusion) is mandatory. EQA samples test ABO/RhD grouping, antibody screening, and antibody identification. Persistent failures trigger corrective action and may require revalidation of staff competency.

Automation

Fully automated platforms perform sample identification (barcode), pipetting, incubation, centrifugation, image analysis, and result transfer to the laboratory information system (LIS). Benefits include:

• Elimination of transcription errors
• Standardised technique and reproducibility
• Audit trail for every test
• Capacity for high-throughput testing in transfusion centres

Special Transfusion Situations

Neonates and Infants (First 4 Months)

• Maternal IgG antibodies cross the placenta; the neonate's own antibody production is negligible
• ABO reverse group is not performed (no reliable isohemagglutinins)
• Maternal plasma should be used for crossmatching if available
• Blood selected must be compatible with both infant and maternal ABO/RhD groups

Patients on Chronic Transfusion Programmes

• Extended phenotyping before first transfusion is essential
• Provide antigen-matched blood at minimum for Rh (C, c, E, e, D) and K antigens in sickle cell disease
• Alloimmunisation rates in sickle cell disease reach 20-50% without extended matching; extended matching reduces this substantially

Antenatal Serology

• ABO, RhD grouping and antibody screening at booking and 28 weeks gestation for all pregnant women
• Positive screens require antibody identification and quantification (titre or level in IU/mL for anti-D and anti-c) to guide fetal surveillance
• Anti-D $\geq 4$ IU/mL and anti-c $\geq 7.5$ IU/mL warrant referral for fetal monitoring

Patients with Autoimmune Haemolytic Anaemia

• Direct antiglobulin test (DAT) positive; serological crossmatch may show apparent incompatibility with all units
• Adsorption of autoantibody from plasma is required before alloantibody screen can be interpreted
• Least incompatible blood is not an acceptable standard; genuinely compatible (alloantibody-excluded) blood must be provided

Key Exam Points

• The reverse group is a critical safety check; its absence or discordance must always be investigated
• Kidd antibodies (anti-Jk$^a$, anti-Jk$^b$) are notorious for causing delayed haemolytic transfusion reactions because titres fall below detectable levels between exposures; dosage effect (homozygous cells) is important for detection
• Enzyme treatment destroys Duffy (Fy$^b$) and MNS antigens; this is a diagnostic tool, not a limitation
• Electronic issue requires two concordant historical ABO/RhD groups and a negative current antibody screen; it is not appropriate if any alloantibody has ever been identified
• Molecular genotyping is the method of choice when serological phenotyping is unreliable (recent transfusion, strong autoantibody, rare antigen)
• All IAT tests must be performed at 37°C; cold-reactive antibodies detected only below 30°C are generally not clinically significant
• Anti-K causes haemolytic transfusion reactions and HDFN partly by suppressing erythropoiesis in the fetus, not solely by haemolysis