HLA and transplantation testing

5 DOH Abu Dhabi (HAAD) lab licensing practice questions on HLA and transplantation testing, part of Immunology and Serology. The first 5 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Medium

   How does a virtual crossmatch predict the result of a cell-based crossmatch?

   1. ABy mixing donor cells and recipient serum in a flow cytometer
   2. BBy comparing the donor and recipient ABO and RhD groups
   3. CBy testing donor serum against recipient lymphocytes
   4. DBy comparing recipient HLA antibodies with donor HLA type

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   Answer: D · By comparing recipient HLA antibodies with donor HLA type

   The recipient's HLA antibodies are identified, usually with single-antigen beads, and listed as unacceptable antigens. The donor's HLA type is then checked for any of these antigens; if none is present, the virtual crossmatch is negative. This allows organ allocation and shipment before a physical crossmatch, or sometimes in place of it. Its accuracy depends on complete donor typing and a recent recipient serum.

   • A: Mixing cells and serum in a flow cytometer is a physical flow crossmatch.
   • B: ABO compatibility is checked separately and is not an HLA crossmatch.
   • C: A crossmatch looks for recipient antibody against donor cells, not the reverse.

2. Question 2 · Hard

   A sensitized kidney recipient received rituximab 2 weeks ago. Crossmatch with a deceased donor: CDC crossmatch negative; flow crossmatch T-cell negative, B-cell strongly positive. Single-antigen bead testing finds no donor-specific antibody. What is the most likely cause of the B-cell result?

   1. ARituximab binding to CD20 on donor B cells
   2. BIgM autoantibody to lymphocytes
   3. CDonor-specific HLA class I antibody
   4. DDonor-specific HLA class II antibody

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   Answer: A · Rituximab binding to CD20 on donor B cells

   Rituximab is a chimeric IgG1 antibody against CD20, a B-cell antigen. In the flow crossmatch it coats donor B cells and is detected by the anti-human IgG reagent, giving a false-positive B-cell result while T cells stay negative. The single-antigen beads carry no CD20 and find no donor-specific HLA antibody, which supports drug interference. Recent therapeutic antibodies should always be recorded on the crossmatch request.

   • B: The flow crossmatch detects bound IgG, and an IgM autoantibody would affect T cells as well.
   • C: Class I antibody would also make the T-cell crossmatch positive and appear on beads.
   • D: Class II donor-specific antibody would show on single-antigen beads.

3. Question 3 · Medium

   Before a complement-dependent cytotoxicity crossmatch, the recipient's serum is sometimes treated with dithiothreitol (DTT). The aim is to:

   1. ARemove immune complexes that hide HLA
   2. BInactivate IgM, often harmless autoantibody
   3. CConcentrate donor-specific IgG antibody
   4. DActivate complement to improve cell killing

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   Answer: B · Inactivate IgM, often harmless autoantibody

   DTT breaks the disulfide bonds that hold IgM pentamers together, abolishing their activity while leaving IgG largely intact. Many positive CDC crossmatches are caused by IgM lymphocytotoxic autoantibodies, which are usually not harmful to the graft. If a positive crossmatch becomes negative after DTT, an IgM antibody was responsible; if it stays positive, IgG, more likely donor-specific HLA antibody, is present. An autologous crossmatch helps confirm an autoantibody.

   • A: DTT is not used to dissociate immune complexes from cells.
   • C: DTT inactivates IgM; it does not concentrate IgG.
   • D: DTT does not activate complement; rabbit complement is added separately.

4. Question 4 · Easy

   The genes of the HLA system (the major histocompatibility complex) lie on the short arm of chromosome:

   1. A15
   2. B6
   3. C9
   4. D1

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   Answer: B · 6

   The HLA genes form the major histocompatibility complex on chromosome 6 (6p21.3). They are closely linked and usually inherited together as a haplotype, one from each parent, and the antigens are expressed codominantly. Beta-2 microglobulin, the light chain of class I molecules, is encoded separately on chromosome 15. The ABO gene is on chromosome 9 and the RH genes are on chromosome 1.

   • A: Chromosome 15 carries beta-2 microglobulin, not the HLA genes.
   • C: Chromosome 9 carries the ABO gene.
   • D: Chromosome 1 carries the RHD and RHCE genes.

5. Question 5 · Easy

   In HLA nomenclature, the asterisk in a type such as HLA-A*02:01 shows that it was defined by:

   1. ASerologic lymphocytotoxicity typing
   2. BMixed lymphocyte culture
   3. CSingle-antigen bead antibody testing
   4. DDNA-based (molecular) typing

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   Answer: D · DNA-based (molecular) typing

   The asterisk marks an allele name defined by DNA sequence, and each field separated by a colon adds detail: the first field (A*02) is the allele group, which usually matches the serologic antigen, and the second (:01) is the specific protein. Serologic types are written without an asterisk, for example HLA-A2. Molecular typing by PCR with sequence-specific primers or probes, or by sequencing, has largely replaced serologic typing. High-resolution typing matters most for stem cell transplant matching.

   • A: Serologic types are written without an asterisk, for example HLA-A2.
   • B: Mixed lymphocyte culture measures cellular reactivity and gives no allele name.
   • C: Single-antigen beads identify antibodies, not the person's own HLA type.