A humanized, non-Fc receptor (FcR)-binding IgG2 monoclonal antibody (MoAb) directed against CD3 with potential immunosuppressive activity. Visilizumab binds to invariant CD3 epsilon, one of the non-covalently-associated subunits of T-cell receptors (TCRs) on activated T-cells. Upon binding to the TCR/CD3 complex, visilizumab induces apoptosis, which may result in the selective clonal deletion of activated pathogenic T-cells. This MoAb is engineered with a substitution at amino acid residues 234 and 237 (Val3Ala) within the IgG2 Fc arm, rendering it unable to bind to type II FcRs; accordingly, this agent is less likely to activate type II FcR-expressing resting T-cells. or A substance being studied in the treatment of an immune system reaction called graft-versus-host disease (GVHD), which may occur after a donor stem cell transplant. It is also being studied in the treatment of some autoimmune diseases. Visilizumab binds to a protein called CD3, which is found on some T cells (a type of white blood cell). This may help suppress the body’s immune response. Visilizumab is a type of monoclonal antibody.