A living donor is sought for a child needing a kidney transplant. The best odds of finding an MHC-compatible donor occur between the child and

• A. A sibling (brother or sister)
• B. An unrelated individual
• C. The child's father
• D. The child's mother

Answer: (A)

Matching HLA (MHC) antigens is most favorable when the donor is a close relative, because relatives share inherited gene segments. A sibling inherits HLA from the same two parents as the patient, giving them the best chance of a compatible set. In fact, a full sibling has about a 25% chance of being HLA-identical to the child, and generally a higher likelihood of sharing one or more matching haplotypes compared with a parent or an unrelated donor. Parents and unrelated individuals have lower odds of providing a near-perfect HLA match, since they don’t share the same combination of haplotypes as the patient as often as a sibling does. So the best odds for an MHC-compatible donor come from a brother or sister.