Xenograft

Xenografts are grafts transplanted between individuals of different species (e.g., pig to human). They are distinguished from autografts (self to self), isografts (between genetically identical individuals, e.g., twins), and allografts (between genetically different individuals of the same species). Xenografts elicit the strongest immune response due to significant antigenic disparity, primarily hyperacute rejection mediated by preformed antibodies against galactose-alpha-1,3-galactose (alpha-gal) epitopes on donor endothelium, leading to complement activation, thrombosis, and graft necrosis within minutes to hours. Clinically, xenografts are most commonly used in a modified, decellularized or chemically treated form to minimize immunogenicity—examples include bioprosthetic heart valves (porcine or bovine pericardial valves) and skin grafts (porcine skin) used temporarily for burn wound coverage. These bioprosthetic valves are less durable than mechanical valves but do not require long-term anticoagulation, making them preferable in elderly patients or those with contraindications to anticoagulation. Research into whole-organ xenotransplantation (e.g., genetically modified pig hearts/kidneys) aims to address organ shortage, requiring genetic knockout of alpha-gal and other xenoantigens plus immunosuppression to prevent hyperacute and acute vascular rejection. Key boards testing points: differentiate autograft/isograft/allograft/xenograft, understand hyperacute rejection mechanism (preformed antibody + complement), and recognize clinical use of bioprosthetic valves (xenografts) versus mechanical valves and their respective anticoagulation requirements.