IgG4-Related Appendiceal Disease: A First Case Report Fulfilling All Pathological Diagnostic Criteria and With Concomitant S100-Positive Dendritic/Schwann Cell Hyperplasia.

IgG4-related disease is a recent entity that has been described in a wide variety of organ systems. A 46-year-old female presented with acute appendicitis accompanied by a mass-forming lesion, raising a concern for neoplasm, and therefore, hemicolectomy was performed. The lesion revealed a dense lymphoplasmacytic infiltrate accompanied by storiform fibrosis and obliterative phlebitis. The IgG4/IgG plasma cell ratio was >50%, and the number of IgG4-positive plasma cells was >100/high-power field. In order to assess the IgG4/IgG plasma cell ratio, MUM1 was employed instead of IgG to successfully estimate the plasma cell concentration. There was also a concomitant hyperplasia of S100-positive cell, which could represent dendritic or Schwannian origin and possibly play a pathophysiologic role. The hyperplasia was significant by itself that it may mimic a mass-forming lesion. This newly described entity of the past decade deserves increased recognition due to clinical implication and surgical morbidity. This is the first case of IgG4-related disease in the appendix to our knowledge that fully satisfied all the pathological diagnostic criteria. We would like to also highlight our innovative approach of evaluating the IgG4/IgG plasma cell.