Hyperacute transplant rejection is characterized by which mechanism and timing?

• A. Acute rejection: CD8+ T cells reacting against foreign MHCs
• B. Graft-versus-host disease: donor T cells attack host
• C. Chronic rejection: fibrosis months later
• D. Hyperacute rejection: preformed antibodies (type II) - occurs in minutes

Answer: (D)

Hyperacute rejection hinges on preformed antibodies against donor antigens, a type II hypersensitivity reaction. In the recipient, circulating antibodies (often against donor HLA or ABO antigens) bind the graft endothelium and activate complement, triggering rapid thrombosis of graft vessels and ischemic graft failure. Because these antibodies are already present before transplantation, the reaction happens within minutes to hours after the transplant. This immediate, antibody-mediated process distinguishes it from other forms: acute rejection is mainly T cell–mediated against donor MHCs and occurs days to weeks later; graft-versus-host disease involves donor T cells attacking the recipient, typically after hematopoietic stem cell transplantation; chronic rejection develops months later with fibrotic changes.