Acute rejection is graft destruction after transplantation and has the following characteristics: Occurs later, about 5 days after transplantation, and unlike hyperacute and accelerated rejection, acute rejection is mediated by a de novo anti-graft T-cell response, not by preexisting antibodies Is caused by a T cell-mediated delayed hypersensitivity reaction to allograft histocompatibility antigens Is characterized by mononuclear cellular infiltration, with varying degrees of hemorrhage, edema, and necrosis but with vascular integrity usually maintained (although vascular endothelium appears to be a primary target) Acute rejection accounts for about half of all rejection episodes that occur within 10 years.