A humanized monoclonal antibody directed against the human immunosuppressive molecule CD200 (OX-2) with potential immunomodulating and antineoplastic activities. Samalizumab binds to CD200, blocking the binding of CD200 to its receptor, CD200R, present on cells of the macrophage lineage; inhibition of CD200 may augment the cytotoxic T-lymphocyte (CTL) mediated immune response against CD200-expressing tumor cells. CD200 is a type 1a transmembrane protein, related to the B7 family of co-stimulatory receptors, and is upregulated on the surface of multiple hematologic malignant cells; this transmembrane protein appears to be involved in the downregulation of a Th1 (helper T cell) immune response.