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CD200, also known as OX-2, is a type I membrane glycoprotein belonging to the immunoglobulin superfamily. It is widely expressed on various cell types, including B cells, T cells, dendritic cells, neurons, and endothelial cells. CD200 primarily functions through its interaction with CD200R, a receptor expressed on myeloid cells, T cells, and other immune cells. This interaction suppresses T-cell-mediated immune responses, reduces the production of inflammatory cytokines, and promotes immune tolerance. In the context of cancer, CD200 expression on tumor cells can lead to immune evasion by inhibiting antitumor T-cell responses. Additionally, CD200 plays a crucial role in maintaining immune homeostasis and is involved in various autoimmune and inflammatory diseases.