Ans: C (Vessel thrombosis) HYPERACUTE REJECTION Harshmohan 7th/50Appears within minutes to hours of placing the transplantSt is mediated by preformed humoral antibodyThere are numerous neu trophils around dilated & obstructed capillaries which are blocked by fibrin & platelet thrombiIn acute rejection there is infiltration ofT&B cell"- Harshmohan 7th/ 50HYPERACUTE REJECTION; Robbins9th/233This form of rejection occurs within minutes or hours after transplantation and can sometimes be recognized by the surgeon just after the graft vasculature is anastomosed to the recipient's.In contrast to the nonrejecting kidney graft, wrhich rapidly regains a normal pink coloration and normal tissue turgor and promptly excretes urine, a hyperacutely rejecting kidney rapidly becomes cyanotic, mottled, and flaccid and may excrete a mere few drops of bloody urine.Immunoglobulin and complement are deposited in the vessel wrall, and electron microscopy discloses early endothelial injury together with fibrin-platelet thrombi.There is also a rapid accumulation of neutrophils within arterioles, glomeruli, and peritubular capillaries. These early lesions point to an antigen-antibody reaction at the level of vascular endothelium.Subsequently, these changes become diffuse and intense, the glomeruli undergo thrombotic occlusion of the capillaries, and fibrinoid necrosis occurs in arterial walls.ACUTE REJECTION. Robbins 9th/233# This may occur within days of transplantation in the untreated recipient or may appear suddenly months or even years later, after immunosuppression has been employed and terminated. As suggested earlier, acute graft rejection is a combined process in wrhicb both cellular and humoral tissue injuries contribute. In any one patient, one or the other mechanism may predominate.Histologically, humoral rejection is associated with vasculitis, whereas cellular rejection is marked by an interstitial mononuclear cell infiltrate.Mechanisms of Recognition and Rejection ofAlografts Robbins 9th/231-32# Rejection is a process in wrhsch T lymphocytes and antibodies produced against graft antigens react against and destroy tissue grafts# Recognition of Graft Alloantigens by T and 8 Lymphocytes: The major antigenic differences between a donor and recipient that result in rejection of transplants are differences in HLA alleles.T CELL-MEDIATED REACTIONS Robbins 9th/233Acute cellular rejection, also called acute T cell-mediated rejection, is most commonly seen within the initial months after transplantation and is heralded by clinical and biochemical signs of organ failure.T cells also contribute to chronic rejection, ANTIBODY-MEDIATED REACTIONS Robbins9th/233Although T cells are pivotal in the rejection of organ transplants, antibodies produced againstalloantigens in the graft are also important mediators of rejection. Antibody mediated reactions can take three forms.Hyperacute rejection occurs when preformed antidonor antibodies are present in the circulation of the recipient. Such antibodies may be present in a recipient who has previously rejected a transplant.Acute antibody-mediated rejection is caused by antidonor antibodies produced after transplantation.Chronic antibody-mediated rejection usually develops insidiously, without preceding acute rejection, and primarily affects vascular components.