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Liu Bai, Jiang Yiqun. Department of Pathology, Institute of Dermatology, Chinese Academy of Medical Sciences and Peking Union Medical College, Nanjing 210042, China Corresponding author: Jiang Yiqun, Email: yiqunjiang@qq.com ��Abstract�� Erythema nodosum, a kind of panniculitis, occurs most frequently on both shins, and is mainly characterized by erythema and painful subcutaneous nodules. It is common in young women. Histologically, it manifests as septal panniculitis complicated by infiltration of various inflammatory cells. The definite pathogenesis remains unclear, and it is currently considered to be caused by delayed hypersensitivity reaction. Some scholars deem that it is caused by neutrophilic panniculitis due to immune complex deposition in small veins in septa between fat lobules. The etiology of erythema nodosum is very complex, and may be associated with infections, systemic diseases, drugs, hormone levels, malignant tumors, etc. Moreover, differences exist in the etiology of erythema nodosum among different countries and areas.