CD200, known as OX-2, is a type I membrane glycoprotein member of the Ig supergene family. CD200 is expressed on B cells, a subset of T cells, thymocytes, follicular dendritic cells, neurons, keratinocytes, vascular endothelium, and some smooth muscle. The interaction of CD200 with CD200 receptor provide a potent costimulatory T-cell signal in the presence of TCR signaling, stimulate macrophages, and inhibit mast cell degranulation. It was reported that increased expression of OX-2 on DC was associated with inhibition of cytokine production and renal allograft rejection. Incubation of lymphocytes with OX-2 Fc inhibits a primary mixed lymphocyte reaction in vitro, decreased IL-2 and IFN-gamma production, increased IL-4 and IL-10 production. In vivo infusion of OX-2 Fc promotes both skin and renal graft survival and decreases the antibody response. The OX-2 antibody reacts with rat OX-2 antigen.