The Fab segment of the antibody targets the CD20 epitope on B-cells, allowing the radioactive yttrium to destroy the cell via production of beta particles.
Ibritumomab is a murine monoclonal antibody against CD20 that has been radiolabeled with yttrium-90.
Most likely removed by opsonization via the reticuloendothelial system when bound to B cells, or by human antimurine antibody production