**1.1 Current status of small bowel transplantation**

Small bowel transplantation (SBT) is one of the standard treatments for patients who are unable to consume a regular diet and have complications from the irreversible requirement of parenteral nutrition [1]. Hirschsprung's disease [2, 3] and Crohn's disease [4] patients are two examples. Recent effective immunosuppressive drugs, well-controlled postoperative care, and advances in diagnostic techniques have significantly improved the outcome of SBT [5]. Immunosuppressants such as mycophenolate mofetil, tacrolimus, and steroids, are routinely used for long-term management after transplantation [6, 7].

Acute cellular rejection (ACR) is a major cause of impaired colonization by the transplanted small intestine, and it frequently accompanies chronic and irreversible changes such as ulcers and lamina propria fibrosis. ACR has remained a risk factor that impedes functional recovery of the intestinal graft [1, 8, 9]. On the other hand, pathologists frequently encounter various pathologies of the intestinal allograft [10–12]. For example, mechanical failure of the graft due to operation during

surgery may occur during the early phase after transplantation. Cytomegalovirus (CMV) enteritis and Epstein–Barr virus -related enteritis are severe side effects and post-transplantation lymphoproliferative disorders/diseases [13–15]. It is often difficult to make a differential diagnosis of the ACR findings. However, histologic diagnosis is critical for the selection of immunosuppressants and their respective doses. Tacrolimus, cyclosporin, and steroids are commonly prescribed in the early stages of rejection [16]. If an excessive dose is administered, the occurrences of CMV enteritis and EBV enteritis become inevitable.

Among the various histological features, crypt epithelial cell apoptosis has been evaluated as a highly reproducible finding. However, other histological findings have been proposed at different institutions. We have also previously suggested other findings as indicators of ACR [17–19].
