The etiology of organ transplant rejection is complex, basically immunological.
Transplanted kidney is recognized as foreign by the patient's immune system.
Grossly, the kidney may vary from unremarkable to necrotic, depending on the extent and stage. The most characteristic changes involve the arterial vasculature. The thickened intima shows inflammatory cell infiltrates and there may be infiltrates of the muscular walls as well. Occlusive vascular thrombosis may be present. Patients with transplanted kidneys are closely monitored and the clinical manifestations will be that of decreasing renal function.