A truncated form of the full-length, immunosuppression-inducing molecule, CD200, CD200.sub.tr, was expressed in CHO cells and the transduced cells used to produce mAbs to CD200.sub.tr. These mAbs detect simultaneous expression of full-length CD200 and CD200.sub.tr in LPS-stimulated dendritic cells (DCs). Moreover, CHO cells and DCs expressing CD200.sub.tr as well as soluble CD200.sub.tr are able to inhibit suppression of mixed leucocyte reactions in vitro which follows addition of the soluble form of CD200 (CD200:Fc) in culture, demonstrating that CD200.sub.tr is a physiological antagonist of CD200.