What is the primary cause of hyperacute rejection in organ transplantation?

Hyperacute rejection is characterized by a rapid and severe immune response to transplanted organs that occurs within minutes to hours after transplantation. The primary cause of this type of rejection is the presence of preformed anti-donor antibodies in the recipient's circulation. These antibodies can recognize and bind to antigens on the surface of the donor's endothelial cells, leading to immediate activation of the complement system and recruitment of inflammatory cells. This process results in thrombosis, vascular damage, and ultimately, graft failure.

The presence of preformed antibodies is often due to prior sensitization of the recipient, which can occur from previous transplants, blood transfusions, or pregnancies where the recipient was exposed to the donor's antigens. Because these antibodies are already in circulation at the time of transplantation, they can cause hyperacute rejection almost immediately, distinguishing it from other forms of rejection which typically take longer to develop.

Understanding hyperacute rejection emphasizes the importance of pre-transplant immunological assessment, including crossmatching, to identify and mitigate potential incompatibilities before transplantation occurs.