Background：CD200, also known as OX2, is a member of the immunoglobulin superfamily （IgSF）. It is a monomorphic cell surface glycoprotein that is expressed on thymocytes, neurons, endothelium, follicular dendritic cells in all lymphoid organs, a subset of CD34+ progenitor cells, and at low levels on some smooth muscle and B lymphocytes. It is not expressed on NK cells, monocytes, granulocytes, or platelets. CD200 costimulates T cell proliferation. It may regulate myeloid cell activity in a variety of tissues. The interaction between CD200 （OX2） and CD200 receptor （OX2R） system is of importance in the control of macrophage and granulocyte activation, which may contribute to pathways that suppress and limit macrophage induced inflammatory damage in tissue.
