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Why can a positive crossmatch rule out a potential organ transplant?

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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.

A positive crossmatch indicates that the recipient’s serum contains preexisting antibodies that react against the antigens of the potential donor.12

In the context of transplant surgery, this result is critical because it signals a high risk of hyperacute rejection. Unlike cell-mediated rejection, which develops over days or months, antibody-mediated rejection occurs rapidly—within minutes or hours—and is generally irreversible. This sensitization often arises from prior exposure to foreign antigens through blood transfusions, previous transplants, or pregnancy.23

Because a positive crossmatch confirms that the recipient’s immune system is already primed to launch a swift and destructive attack on the donor tissue, it is considered a contraindication for the procedure to ensure patient safety and prevent immediate graft failure.12