Acute Cellular Rejection - Grade 0R
Morphologically, acute cellular rejection consists of a mononuclear inflammatory infiltrate that is predominantly a T-cell mediated response directed against the cardiac allograft. In severe cases, there is also participation of granulocytes in the rejection process. B cell infiltrates are rarely present in mild rejection. However, a substantial increase in activated B lymphocytes and NK cells are seen in moderate rejection, suggesting their important role as promoters and effectors of cellular rejection.
The absence of cellular rejection is called Grade 0R and and examples are shown below. The biopsy examples shown below were classified as Grade 0R since there is no evidence of mononuclear
(lymphocytes / macrophages) inflammation or myocyte
damage. 
At higher magnification the following biopsy images show the absence of mononuclear cells in the interstitial space is clear. Also note the endothelial cells of the capillaries showing no evidence of cytoplasmic or nuclear swelling.
