A 35-year-old woman with end-stage renal disease of unknown etiology is transplanted with a cadaver kidney. The patient develops oliguia shortly after transplantation and a renal biopsy shows immediate (hyperacute) rejection. Immunosuppression improves renal function. Which of the following represents the principle target for immune attack directed against this patient's allograft?

Incompatible ABO histo-blood group antigens, which are expressed on endothelial cells and erythrocytes, are absolute barriers to a successful transplant. ABO-incompatible grafts encounter preformed circulating antibodies, which bind to endothelial cells and cause immediate (hyperacute) rejection. By contrast, the most common patterns of acute and chronic rejection are caused primarily by donor-recipient differences in HLA molecules encoded by the major histocompatibility complex. These molecules are expressed on most cell surface membranes. Other causes of transplant rejection tend to be chronic, because they do not involve preformed antibodies. None of the other choices mediates hyperacute graft rejection.Diagnosis: Graft-versus-host disease