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CD200, also known as OX-2, is a member of the immunoglobulin superfamily and a type I membrane glycoprotein encoded by the CD200 gene. It is widely expressed on various immune and non-immune cells, including T cells, B cells, dendritic cells, endothelial cells, and neurons. CD200 plays a crucial role in immune regulation by interacting with its receptor, CD200R, which is primarily expressed on myeloid cells, such as macrophages, and some lymphoid cells. This interaction transmits inhibitory signals, downregulating immune responses, reducing the production of inflammatory cytokines, and expanding regulatory T cells. CD200 is also involved in maintaining immune homeostasis and preventing excessive immune activation and autoimmune diseases. Additionally, it is associated with immune evasion in certain cancers, as its expression on tumor cells can suppress anti-tumor immune responses.