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CD200 （OX2） protein is a type-1 membrane glycoprotein belonging to the immunoglobulin superfamily, widely distributed on various cell types including thymocytes, neurons, endothelial cells, and immune cells such as macrophages and dendritic cells. It plays a crucial role in immune regulation by interacting with its receptor CD200R, which is highly expressed on myeloid cells and some lymphoid cells. The CD200-CD200R interaction inhibits inflammatory responses, promotes immune tolerance, and helps maintain tissue homeostasis. In cancer, CD200 is often overexpressed on tumor cells, contributing to immune evasion by suppressing anti-tumor immune responses. Additionally, elevated levels of soluble CD200 （sCD200） in serum are associated with adverse tumor prognosis in several cancers.