**3.1 Hyperacute rejection**

In hyperacute rejection, the transplanted tissue is rejected within minutes to hours after graft implantation because transplant patients are serologically presensitized to nonself graft antigens, which are known as alloantigens. Histologically, numerous polymorphonuclear leukocytes (PMNs) exist within the graft vasculature and are associated with widespread microthrombi formation and platelet accumulation. Little or no leukocyte infiltration occurs. Hyperacute rejection is humorally mediated and occurs because the recipient has preexisting antibodies against graft-derived antigens, which can be induced by prior blood transfusions, multiple pregnancies, prior transplantation, or xenografts. The antigenantibody complexes activate the complement system, causing massive thrombosis in the capillaries, which prevents graft vascularization. The liver is relatively resistant to hyperacute rejection. Although this may be due to its dual blood supply, it is more likely because of incompletely understood immunologic properties. Hyperacute rejection has become relatively rare since the introduction of routine pretransplantation screening of graft recipients for anti-donor antibodies.
