Histocompatibility, Crossmatch

The contract between this chapter and the reader.

1. 1. Describe HLA typing and its resolution.
2. 2. Explain antibody screening, PRA, and calculated PRA.
3. 3. Detect and quantify anti-HLA antibodies with single-antigen beads.
4. 4. Describe the complement-dependent and flow cytometry crossmatches.
5. 5. Explain the virtual crossmatch and its requirements.
6. 6. Interpret a positive crossmatch, including false positives.
7. 7. Define sensitization and its causes and consequences.
8. 8. Stratify immunologic risk for a transplant.
9. 9. Recognise the uncertainty in DSA thresholds and crossmatch methods.

A sixty-second reading. Each bullet stands alone.

• Histocompatibility testing types the donor and recipient HLA and detects the recipient's anti-HLA antibodies.
• HLA typing is now molecular (DNA-based), at low or high resolution.
• Antibody screening reports panel-reactive antibody; calculated PRA estimates the proportion of donors a patient would be incompatible with.
• Single-antigen bead assays identify antibody specificities and semi-quantify them by mean fluorescence intensity.
• The crossmatch tests recipient serum against donor cells for reactive antibody.
• A positive T-cell complement-dependent crossmatch indicates high-titre donor-reactive antibody and contraindicates transplant (hyperacute risk).
• The flow cytometry crossmatch is more sensitive and detects lower antibody levels.
• The virtual crossmatch predicts compatibility from the recipient's defined antibodies and the donor's HLA type, without cells.
• Autoantibodies and non-HLA antibodies can cause a false-positive crossmatch, which must be distinguished.
• Sensitization — from prior transplant, pregnancy, or transfusion — raises calculated PRA and shrinks the compatible donor pool.
• Highly sensitized patients wait longer, are prioritised in allocation, and are at higher antibody-mediated rejection risk.
• Immunologic risk is stratified from DSA, calculated PRA, matching, and history to set induction and monitoring intensity.
• The mean-fluorescence-intensity threshold for a clinically significant DSA is not absolute and is laboratory-dependent.
• The virtual crossmatch is only as good as the HLA typing and antibody definition behind it.

The medical core. An expert should agree histocompatibility testing is fully covered here.

Why it matters at the bedside

The previous chapter explained why the immune system attacks the graft; this one is how we measure that threat before we transplant. Typing, antibody screening, and the crossmatch together answer a single question — will this recipient's immune system tolerate this donor — and getting the answer wrong, in either direction, costs a graft or a chance at one.

HLA typing

• Both donor and recipient are typed for the key HLA loci — A, B, C, DR, DQ (and DP). Once done by serology, typing is now molecular (DNA-based), reported at low resolution (broad antigen groups) or high resolution (specific alleles). High-resolution typing matters for precise matching and for interpreting which antibodies are donor-specific.

Antibody screening: PRA and cPRA

• The recipient's serum is screened for anti-HLA antibodies. The older panel-reactive antibody expressed reactivity as a percentage of a test panel; the modern calculated PRA (cPRA) uses the recipient's defined antibody specificities and population HLA frequencies to estimate the percentage of donors that would be incompatible — a direct measure of how hard the patient will be to match.

Single-antigen beads and MFI

• Solid-phase single-antigen bead assays (Luminex) are the core tool: each bead carries one HLA antigen, so the assay maps exactly which specificities a patient has antibody against, semi-quantified by mean fluorescence intensity. From these specificities, the laboratory lists ‘unacceptable antigens’ and computes the cPRA.

The crossmatch: CDC and flow

• The crossmatch puts recipient serum against donor cells to see whether antibody binds. In the complement-dependent cytotoxicity (CDC) crossmatch, bound complement-fixing antibody lyses donor lymphocytes — a positive T-cell CDC crossmatch signals high-titre donor-reactive antibody and contraindicates transplant for fear of hyperacute rejection. The flow cytometry crossmatch is more sensitive, detecting lower antibody levels against T and B cells.

The virtual crossmatch

• When the recipient's antibody specificities and the donor's HLA type are both well defined, compatibility can be predicted without any cells — the virtual crossmatch. By comparing the recipient's unacceptable antigens against the donor's type, it enables rapid, accurate allocation (especially for deceased-donor offers) before a physical crossmatch is even possible.

Interpreting a positive crossmatch and false positives

• A positive crossmatch is not automatically a contraindication. Autoantibodies and non-HLA antibodies can lyse or bind donor cells without being donor-specific, producing a false-positive result; an autocrossmatch and treatments such as dithiothreitol help distinguish them. The crossmatch is always interpreted alongside the defined antibody profile — a positive crossmatch with no corresponding DSA points away from a true alloantibody.

Sensitization and its consequences

• Sensitization — exposure to foreign HLA through a prior transplant, pregnancy, or transfusion — generates anti-HLA antibodies and a high cPRA. The practical consequences are stark: fewer compatible donors, longer waiting, prioritisation for highly sensitized candidates in allocation schemes, and, if transplanted, a higher risk of antibody-mediated rejection.

Immunologic risk stratification

• The tests combine into a risk grade. A patient with no DSA, low cPRA, and a good match is low immunologic risk and receives standard induction; a patient with DSA, high cPRA, a re-transplant, or ABO-incompatibility is high risk and warrants stronger (lymphocyte-depleting) induction, closer monitoring, and sometimes desensitization. The lab result becomes a clinical plan.

The uncertainty: thresholds and methods

• Precision has limits. The mean-fluorescence-intensity value that defines a ‘clinically significant’ DSA is laboratory-dependent and confounded by technical effects (such as the prozone phenomenon), so no single number is definitive. The virtual crossmatch is only as reliable as the typing and antibody data behind it, and the added value of complement-binding assays is debated — which is why histocompatibility is read by experts, not by a cutoff.

Five fully-built tables.

Table A — The tests

Table B — Crossmatch methods

Table C — Sensitization

Table D — Interpreting the crossmatch

Table E — Immunologic risk strata

Visualise & Map

Causal chains, each ending in a named action.

Chain 1 — The complement-dependent crossmatch

Recipient anti-HLA antibody + donor cells → complement-mediated lysis → positive CDC crossmatch → ACTION: do not transplant (hyperacute risk).

Chain 2 — The virtual crossmatch

Defined unacceptable antigens + donor HLA type → predicted incompatibility → virtual crossmatch → ACTION: use it for rapid, accurate allocation.

Chain 3 — Sensitization

Sensitizing event → anti-HLA antibodies → high cPRA → fewer compatible donors → ACTION: prioritise and strategise allocation (and consider desensitization).

Chain 4 — The false-positive crossmatch

Autoantibody binds self/donor cells → crossmatch positive without DSA → false positive → ACTION: distinguish with an autocrossmatch / DTT before excluding the donor.

Chain 5 — From DSA to risk

Donor-specific antibody present → antibody-mediated rejection risk → higher immunologic risk → ACTION: intensify induction and monitoring.

Numbered rules. These numbers are the cross-reference handle for the cases and flowcharts.

Clinical Reasoning

Five cases. Each stops you at a decision before it answers it.

Every mechanism from Level 3 earns a bedside consequence and an action.

Exhaustive. Every rule in the chapter is here.

Safety & Evidence

Panel A — Red flags

Panel B — NEVER DO

Anti-patterns clinicians fall into. Each becomes a Level 22 distractor.

The grade reflects strength of evidence, not importance.

Patient Decisions

Outcomes as natural frequencies. Figures are representative; the direction of effect is given where precise numbers are uncertain.

Apply & Test

Pre-rounds compression. Rules only.

Active recall. At least one card per objective.

Micro-teaching for rounds. Two scenarios, five steps each.

Metacognition anchored to this chapter's tensions. No answers provided.

1. 1. Our assays detect antibody with ever-greater sensitivity; how do you decide which of those antibodies actually threatens a graft, and which is noise?
2. 2. A high cPRA can mean years of waiting; how do you weigh the fairness of prioritising the highly sensitized against everyone else on the list?
3. 3. The virtual crossmatch is fast but only as good as its data; when is the time saved worth the risk of an imperfect prediction?
4. 4. A borderline MFI invites both over- and under-reaction; how do you hold a number lightly without ignoring it?
5. 5. Refusing a donor on a positive crossmatch protects against hyperacute rejection but costs a chance at transplant; how do you weigh a false positive against a missed opportunity?

Nine items, each anchored in this chapter. At least one per objective.