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How do antibodies cause rapid transplant rejection?

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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.

Rapid transplant rejection occurs when a recipient possesses preexisting antibodies that recognize antigens on the donor tissue. These antibodies may be present due to prior exposure through pregnancy, blood transfusions, or previous transplants.1

Unlike the slower, cell-mediated rejection process, antibody-mediated rejection happens within minutes or hours and is generally irreversible. When these antibodies bind to the donor's antigens, they trigger the classical pathway of the complement system. This cascade of proteins leads to the formation of a membrane attack complex, which inserts itself into the membranes of the donor's cells, causing them to burst (lysis). Additionally, the activation of the complement system releases fragments that promote a severe inflammatory response, further damaging the transplanted tissue.12