Extraordinary Case ReportCase of Generalized Tumor-Type Rosai–Dorfman Disease With Sarcoidosis-Like Histological Features and IgG4-Positive Plasma CellsTan, Yixin MD, PhD; Zhou, Ying MD, PhD; Zhan, Yi MD, PhD; Luo, Shuaihantian MD, PhD; Liu, Yu MD, PhD; Zhang, Guiying MD, PhD Author Information Department of Dermatology, the Second Xiangya Hospital, Central South University, Changsha, China. Correspondence: Guiying Zhang, MD, PhD, Department of Dermatology, the Second Xiangya Hospital, Central South University, No. 139 Ren Min Middle Road, Changsha 410011, China (e-mail: [email protected]). Y. Tan is currently receiving a grant (#81703131) from the National Natural Science Foundation of China. The authors declare no conflicts of interest. The American Journal of Dermatopathology: January 2021 - Volume 43 - Issue 1 - p e9-e12 doi: 10.1097/DAD.0000000000001724 Buy Metrics Abstract Rosai–Dorfman disease (RDD, also known as sinus histiocytosis with massive lymphadenopathy) is a rare and benign non–Langerhans cell histiocytosis. Skin biopsy usually shows nodular or diffuse dermatitis. Rosai–Dorfman cells (RDD cells) and emperipolesis are the key to diagnosis. RDD cells express S-100 antigen, CD68, CD163, α1-antitrypsin, α1-antichymotrypsin, and ham-56, whereas Langerhans cell markers such as CD1a and langerin are negative. We presented a case of a 55-year-old man with varying sizes of many dark red nodules and lumps over the face, trunk, and limbs for approximately 1 year but without systemic involvement. The results of the laboratory evaluations were notable for an increased level of serum IL-6 and serum IgG4. Histopathological examination showed a diffused dense nodular infiltration of “nude” epithelioid histiocytes with infiltration of minimal lymphocytes and plasm cells around the epithelioid nodules. Immunohistochemistry identified nodular histiocytes being stained strongly positive for S-100 and CD68 but negative for CD1a. Plasma cells showed focally positive for IgG, IgG4, and CD38 and with a ratio of IgG4/IgG >40%. Considering these findings, we believe that our case meets the diagnostic description of “cutaneous Rosai–Dorfman disease” and is, therefore, a rare case with clinical features of multiple tumor-like nodules, sarcoidosis-like histological features, and immunohistochemistry of IgG4-positive plasma cells. Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved.