The characteristic feature of severe rejection is the presence of areas of multicellular parenchymal necrosis. The latter is associated with mixed inflammation including numerous neutrophils, and often all of the features described described for grades II, III and IV. Rarely there is extensive coagulation necrosis and it is difficult to evaluate subtle morphological features. In cases of necrosis the differential diagnosis should include post-transplant lymphoproliferative disorders and ischemia.
Ischemia is manifested in the acinar parenchyma as pale patches of coagulation necrosis that occasionally involve clusters of only few cells. In these cases there are no significant inflammatory infiltrates with the exception of some neutrophils. And there is often associated some fat necrosis.
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