What causes hyperacute transplant rejection?

Hyperacute transplant rejection is primarily caused by pre-existing host antibodies that react against donor antigens. When a transplanted organ is introduced into a recipient who has these antibodies, typically due to prior sensitization (from previous transplants, blood transfusions, or pregnancies), an immediate and severe immune response is triggered. This response occurs within minutes to hours after the transplant, leading to rapid graft failure.

The reaction involves complement activation and the formation of immune complexes, which result in inflammation, endothelial damage, and thrombosis in the transplanted organ. This type of rejection is distinct because it is not mediated by T cells or delayed-type hypersensitivity, which are characteristic of acute rejection processes. It emphasizes the importance of matching donor and recipient blood types and HLA antigens to prevent such immediate and devastating outcomes.