Which donor would be expected to elicit a positive mixed lymphocyte reaction in lymphocytes from a patient who has the HLA-DRB1*01:01, 01:03 alleles?

• A. DRB1*01:01, 01:03
• B. DRB1*01:01, 01:01
• C. DRB1*01:03, 01:03
• D. DRB1*01:01, 01:05

Answer: (C)

Mixing lymphocytes from a patient with donor cells tests whether the donor’s MHC class II molecules are recognized as foreign. A positive response occurs when the donor expresses at least one HLA-DRB1 allele that the patient does not have, because that non-self MHC is seen as foreign by the patient’s T cells.

The patient has HLA-DRB1 alleles 01:01 and 01:03. Evaluating donors:

• Donor with 01:01 and 01:03: both alleles are present in the patient → no non-self MHC → no response.

• Donor with 01:01 and 01:01: both alleles are present in the patient → no non-self MHC → no response.

• Donor with 01:03 and 01:03: both alleles are present in the patient → no non-self MHC → no response.

• Donor with 01:01 and 01:05: 01:05 is not in the patient → non-self MHC → positive response.

Therefore, the donor carrying an allele not found in the patient (01:05) would elicit a positive mixed lymphocyte reaction.