Ocrelizumab is an investigational humanized monoclonal antibody that selectively binds to a particular protein - the CD20 antigen - on the surface of B-cells, which are believed to play a critical role in immunologic diseases. Ocrelizumab binds to and eliminates CD20-positive B-cells. Stem cells (B-cell progenitors) in bone marrow lack the CD20 antigen, allowing healthy B-cells to regenerate after treatment and eventually return to normal levels. Ocrelizumab is also being studied in patients with multiple sclerosis.