An early post-transplant lymphoproliferative lesion characterized by immunoblastic proliferation and paracortical expansion as seen in infectious mononucleosis. There is architectural preservation of the involved tissues. Oligoclonal or monoclonal cellular populations have been detected in a small number of cases. In some cases, a polymorphic or monomorphic post-transplant lymphoproliferative disorder may follow this early lesion. [from
NCI]