Pathophysiology
Clinical meaning
Transplant rejection occurs when the recipient's immune system recognizes donor organ tissue as foreign (non-self) through human leukocyte antigen (HLA) mismatches and mounts an immune response to destroy the graft. Hyperacute rejection occurs within minutes to hours due to pre-formed antibodies against donor HLA antigens, causing immediate vascular thrombosis and graft necrosis. Acute rejection occurs days to months post-transplant through T-cell mediated (cellular) or antibody-mediated (humoral) mechanisms, presenting with organ-specific dysfunction. Chronic rejection develops over months to years through progressive vascular changes, interstitial fibrosis, and gradual organ dysfunction. Prevention requires lifelong immunosuppressive therapy, typically a calcineurin inhibitor (tacrolimus or cyclosporine), an antiproliferative agent (mycophenolate), and corticosteroids. Balancing rejection prevention with infection and malignancy risk from immunosuppression is the central challenge of transplant care.