Acute cellular rejection (ACR) occurs in 30-60% of orthotopic liver transplants. It can be noted late in the first week following transplant, or up to many years later; the pattern of LFT abnormalities is unpredictable but is usually hepatocellular in nature. At this time, liver biopsy is the diagnostic standard.
There is a triad of findings in ACR: mixed inflammatory cell infiltrate (lymphocytes, neutrophils and eosinophils, in the portal +/- lobular space), cholangiolitis (inflammation of the bile ducts) and endothelialitis (inflammation of the portal vein).
In this histologic specimen, notice two things. First, it is apparent that there are far too many nuclei surrounding the portal triad (zone 1), as compared to the more sparse hepatocyte nuclei out in the lobule (zone 3). This is the mixed inflammatory cell infiltrate. Second, the arow is pointing to endothelialitis. Notice the inflammatory cells lifting up the vascular basement membrane and pushing it into the lumen (or just trust me). The overall degree of inflammation will determine whether the episode of ACR is mild, moderate or severe.