hyperacute transplant rejection

Hyperacute rejection is a type II hypersensitivity reaction mediated by preformed antidonor antibodies present in the recipient's circulation before transplantation. These antibodies are typically directed against ABO blood group antigens or donor HLA Class I molecules, often from prior transplants, blood transfusions, or pregnancy. Upon reperfusion of the graft, these antibodies bind to donor vascular endothelium, activating the complement cascade. This triggers endothelial damage, exposure of subendothelial collagen, platelet activation, and fibrin deposition, leading to widespread microvascular thrombosis and ischemic necrosis of the graft. Clinically, the transplanted organ rapidly becomes cyanotic, mottled, and nonfunctional within minutes to hours of vascular anastomosis—surgeons may observe this occurring in real time on the operating table. Histologically, there is evidence of neutrophilic infiltration within vessel walls, fibrin-platelet thrombi, and necrosis, but notably minimal lymphocytic infiltration (distinguishing it from acute cellular rejection). Because hyperacute rejection is antibody and complement mediated rather than cell-mediated, immunosuppressive therapy is ineffective, and the only treatment is immediate surgical removal of the graft. Prevention is the mainstay of management: this includes ABO compatibility testing and crossmatching (mixing recipient serum with donor lymphocytes) prior to transplantation to detect preformed antidonor antibodies. This phenomenon is in contrast to acute rejection (T-cell mediated, occurring weeks to months post-transplant, potentially reversible with immunosuppressants) and chronic rejection (occurs over months to years, involves both humoral and cellular mechanisms, leads to fibrosis and vascular sclerosis, largely irreversible).