Richard French, 53 years old, was diagnosed with chronic myelogenous leukemia. His elder brother Don ...

Rationale: The correct answer is a. An HLA haploidentical donor has one HLA haplotype in common with the recipient and one that is different. A bone marrow sample from a haploidentical donor contains numerous alloreactive T cells that can respond to the HLA class I and class II molecules encoded by the HLA class I and II genes of the recipient's HLA haplotype that is not shared with the donor. These alloreactive T cells have the potential to cause a severe and life-threatening graft-versus-host-disease. To prevent such an outcome, bone marrow grafts from haploidentical donors are purged of T cells before being infused into the recipient. Haploidentical transplants are only given to patients who are unable to find an HLA-matched donor. Family members are good candidate donors because all parents and 50 of siblings (on average) have one HLA haplotype in common and one that is different from a patient such as Richard. Unlike Don, who is HLA-haploidentical to Richard, his other sibling Margaret could not be a donor because neither of her HLA haplotypes is shared with Richard.