Abstract: Li Lei, Xiao Yujuan, Yan Heng, Hao Jin, Diao Qingchun Southwest Medical University, Luzhou 646000, Sichuan, China （Li L）; Department of Dermatology, Chongqing Traditional Chinese Medicine Hospital, Chongqing First People′s Hospital, Chongqing 400011, China （Xiao YJ, Yan H, Hao J, Diao QC） Corresponding authors: Xiao Yujuan, Email: Xiaoyujuan1986@163.com; Hao Jin, Email: 284706197@qq.com 【Abstract】 A 28-year-old female patient presented with indurated erythema and nodules on the right lower limb for 2 years, with mild itching and pain. Skin examination showed a well-circumscribed irregular dark red patch measuring about 10 cm × 5 cm in size on the extensor aspect of the right thigh. On the patch, there were scattered or densely distributed mung bean- to soybean-sized quasi-circular violaceous nodules with a smooth surface, which were hard on palpation. Subcutaneous nodules with medium hardness were found on palpation, and hyperpigmentation was observed on the surface of some nodules. Local skin temperature was increased, with tenderness on palpation. Histopathologically, mononuclear cells showed nodular or sheet-like distribution in the middle and upper dermis, some of which had pale-staining cytoplasm. Moreover, plenty of plasma cells were observed. Immunohistochemistry revealed that histiocytes were stained strongly positive for S100. The number of IgG4-positive plasma cells increased obviously, and was more than 50 per high-power field （× 200）. The proportion of IgG4-positive plasma cells in IgG-positive plasma cells was 45%. Finally, the patient was diagnosed with cutaneous Rosai-Dorfman disease with increased IgG4-positive plasma cells.

Key words: Histiocytosis, sinus, Immunoglobulin G, IgG4-related sclerosing disease