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How does tissue typing help doctors select an organ donor?

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Kidney transplant — transplant British surgeon Andrew Ready flushing a donated kidney during a transplant at Queen Elizabeth Hospital, Birmingham, England, 2006.

Tissue typing is a critical process used to minimize graft rejection by identifying the human leukocyte antigens (HLA) of both a donor and a recipient. These antigens, part of the major histocompatibility complex, act as markers that the immune system uses to distinguish "self" from "nonself" tissue.123

During the procedure, lymphocytes from the donor and recipient are tested against a spectrum of typing sera containing antibodies to specific HLA antigens. By observing whether these cells clump together or die, doctors can determine which antigens the individuals share and which are mismatched.1

In addition to HLA matching, doctors must also group red blood cells to ensure compatibility, as these antigens can also trigger rejection. Finally, a direct crossmatch—mixing the recipient’s serum with donor lymphocytes—is performed; a positive result typically indicates that the transplant should not proceed, as it suggests the recipient has preexisting antibodies that would cause rapid, irreversible rejection.12