One of the earliest morphological changes seen with humoral rejection is platelet-fibrin thrombi in the vasculature. This section was taken 30 minutes after transplantation of a normal liver into a presensitized rat recipient. The titer of preformed antibodies was low enough not to cause precipitous graft failure, but high enough to affect the pace of the rejection reaction. Note the margination of platelet in the central veins. Similar changes are seen in the portal tracts and sinusoids.