CD200 is a transmembrane immunoregulatory protein that belongs to the immunoglobulin superfamily. It contains one Ig like V type domain and one Ig like C2 type domain in its extracelluar domain. CD200 is widely but not ubiquitously expressed. Its receptor (CD200R) is restricted primarily to mast cells, basophils, macrophages, and dendritic cells, which suggests myeloid cell regulation as the major function of CD200. CD200 and CD200R associate via their respective N-terminal Ig-like domains. In myeloid cells, CD200R initiates inhibitory signals following receptor-ligand contact. In T cells, CD200 functions as a co-stimulatory molecule independent of the CD28 pathway. In addition, CD200 also plays an important role in prevention of graft rejection, autoimmune diseases and spontaneous abortion.