2.2 HLA Typing Resolution, Donor Matching, DSA & ABO Compatibility

1. HLA Typing Resolution & Matching Standards

Historically, HLA typing was performed using serological complement-dependent cytotoxicity (CDC) assays, identifying broad antigen groups. Modern transplantation mandates DNA-based molecular typing utilizing Sequence-Specific Oligonucleotide Probes (SSOP), Sequence-Based Typing (SBT), or Next-Generation Sequencing (NGS).

Molecular Nomenclature Breakdown

HLA alleles are designated by a standardized international nomenclature (e.g., HLA-A*02:01:01:02L):

1. HLA-A*: Gene locus (HLA-A).
2. 02: First field (one-field / low-resolution family), historically associated with the serologic antigen family (e.g., HLA-A2).
3. 01: Second field (allele-level / two-field), distinguishing alleles that may encode different protein sequences in the peptide-binding region.
4. 01: Third field, designating synonymous DNA base substitutions within the coding region (no change in amino acid sequence).
5. 02: Fourth field, designating non-coding sequence mutations (introns/promoters).
6. L / N / S / Q / C: Suffix indicating expression status (N = null allele; L = low expression; S = secreted).

Exam Tip: One-field typing is insufficient for core unrelated-donor selection. Use current two-field/allele-level nomenclature rather than the historical shorthand “4-digit,” and apply the transplant program's current matching standard for core and additional loci.

Matching Paradigms in Clinical Practice

• Matched Sibling Donor (MSD): Genotypically identical across chromosome 6; typing confirms identity (often 6/6 or 8/8).
• 8/8 Matched Unrelated Donor (MUD): High-resolution match at HLA-A, -B, -C, and -DRB1.
• 10/10 Matched Unrelated Donor: High-resolution match at HLA-A, -B, -C, -DRB1, and -DQB1.
• 12/12 Matched Donor: Adds high-resolution matching at HLA-DPB1.
• 7/8 or 9/10 Mismatched Unrelated Donor (MMUD): Single locus mismatch (allele or antigen level), which correlates with a ~10% decrement in 5-year overall survival and increased risk of graft rejection and acute GVHD unless mitigated by post-transplant cyclophosphamide (PTCy).

2. Panel Reactive Antibodies (PRA), Donor-Specific Anti-HLA Antibodies (DSA) & Crossmatching

Recipients who have been exposed to foreign HLA antigens through prior blood product transfusions, pregnancies, or previous organ/tissue transplants may develop circulating anti-HLA antibodies (alloimmunization).

Panel Reactive Antibodies (PRA)

PRA measures the percentage of a representative donor panel with which the recipient's serum reacts (0% to 100%). A high PRA (>50–80%) signifies broad alloimmunization, predicting difficulty in locating a compatible donor.

Donor-Specific Anti-HLA Antibodies (DSA)

DSA are recipient antibodies directed against a candidate donor's HLA antigens. They can increase graft-failure risk, especially with mismatched, haploidentical, or cord grafts. Interpret solid-phase signal strength, complement-binding or crossmatch results, locus, titer trend, prior sensitization, graft source, and center assay together; no single fluorescence cutoff is universal across laboratories. If a clinically important DSA is present, the team may select another donor or use a specialist desensitization plan. Nurses coordinate samples, medication timing, apheresis or infusion support, and reassessment without treating one protocol as universal.

3. ABO Incompatibility in Hematopoietic Cell Transplantation

ABO matching is not required for HLA-matched HCT, but incompatibility changes product processing and transfusion support.

• Major mismatch: recipient antibodies target donor red-cell antigens. Marrow products with substantial red-cell content may require red-cell depletion or another laboratory strategy based on isohemagglutinin titer, product volume, and SOP.
• Minor mismatch: donor plasma or passenger lymphocytes can target recipient red cells. Plasma reduction and post-HCT hemolysis surveillance may be used when indicated.
• Bidirectional mismatch: both directions are present and require a phase-specific plan.

Transfusion support changes as recipient hematopoiesis gives way to donor hematopoiesis. Red cells, plasma, and platelets must be selected to avoid clinically important incompatibility with both donor and recipient antigens/antibodies during the transition. The transfusion service—not a memorized group-O/group-AB shortcut—defines the component plan, transition criteria, and exceptions for inventory, titer, platelet additives, and emergency release.

4. The Nursing Workflow Behind Typing, DSA and Compatibility

Histocompatibility results reach the chart as conclusions, but nurses coordinate the specimens and timing that produce them, and errors at this step are consequential and difficult to detect later.

Specimen requirements are assay-specific. Molecular HLA typing, antibody screening, and crossmatching each have defined tube types, minimum volumes, temperature handling, and transport windows set by the histocompatibility laboratory. A tube drawn correctly for a chemistry panel may be unusable for typing. Confirm the requirements against the laboratory's current instructions rather than assuming, and label at the bedside, because a mislabeled histocompatibility specimen can propagate into a donor selection decision.

Confirmatory typing is a distinct event. Programs require a second, independently drawn sample to verify identity and typing before an unrelated donor is cleared and before conditioning begins. It is not a repeat of the same draw and cannot be satisfied by splitting one specimen. Recent transfusion of cellular blood products can also interfere with recipient typing, so the draw is timed with that history in mind.

Antibody results are dynamic. Anti-HLA antibody profiles change after transfusion, pregnancy, infection, and immunologic events, so the team may require re-screening close to the transplant date rather than relying on a result from months earlier. If desensitization is planned, nurses coordinate the sequence of apheresis, antibody-directed therapy, sampling, and infusion, where the timing between steps is the point of the protocol.

5. Phase-Based Component Selection in ABO-Incompatible Transplant

Transfusion support in an ABO-incompatible transplant is conventionally organized into phases, because the compatible component changes as the patient's blood group converts from recipient to donor type.

• Before and immediately after infusion, components must avoid reacting with either the recipient's circulating antibodies or the donor cells that have just been given.
• During the transition, while both recipient antibody and emerging donor red cells are present, the transfusion service selects components compatible with both, which frequently means a component group matching neither the donor nor the recipient.
• After conversion is documented, with donor-type red cells established and recipient isohemagglutinins no longer detectable, component selection can move to donor type.

The point of learning this structure is not to memorize a group for each cell, because the transfusion service defines the phase criteria and the exceptions for inventory, titer, platelet additive volume, and emergency release. The point is that the component plan changes over time on the basis of laboratory evidence, so a nurse who notices an order for an unexpected blood group should confirm the current phase rather than assume an error, and should never accept a component group by memory when the chart carries an active transfusion plan.

Delayed red cell recovery, and occasionally pure red cell aplasia, can follow major ABO mismatch, so a persistent transfusion requirement in a patient whose neutrophils and platelets have recovered is a recognized pattern that warrants reporting rather than routine transfusion.