CD200, also known as OX-2 membrane glycoprotein, is a type I transmembrane protein encoded by the CD200 gene and belongs to the immunoglobulin superfamily. It is primarily expressed on various cell types, including immune cells, neuronal cells, and some non-hematopoietic cells. CD200 plays a crucial role in immune regulation by interacting with its receptor, CD200R, which is mainly expressed on myeloid cells, microglia, and activated T cells. This interaction generates inhibitory signals that suppress immune responses, such as reducing the production of inflammatory cytokines like IL-2 and IFN-纬, and promoting the expansion of regulatory T cells. In addition, CD200 is involved in maintaining immune tolerance and preventing autoimmune diseases. However, its expression on tumor cells can lead to immune evasion by inhibiting antitumor T-cell responses. Therefore, CD200 is considered a potential target for cancer immunotherapy and the treatment of inflammatory and autoimmune diseases.