The antibody binds specifically to CD4，a molecule which is part of the immunolgobulin super-family，expressed on the surface of helper T cells. The CD4 molecule acts as a co-receptor in the activation of T cells，aiding the interaction between the TCR and MHC II of the antigen-presenting cell. The antibody is classified as "primatized"，as the variable regions of the light and heavy chains are obtained from primates，while the lamda light chain and γ4 heavy chain constant domains are human. The antibody was engineered from the primatized IgG1 monoclonal antibody Keliximab by substituting two key residues. The role of the antibody as an immunosuppressor has been tested in vivo，by intravenous administration in chimpanzees and human volunteers. The antibody reached Phase II clinical trials with Biogen-Idec.
