Selective removal of mol. moieties from plasma of patients can result in tumor regression, auto-antibody clearance, and immune modulation. The Prosorba column consists of highly purified protein A isolated from Staphylococcus and covalently linked to inert silica housed in a polycarbonate device. Protein A acts as an immunoadsorbent to selectively remove IgG and IgG-containing CIC via specific binding to the Fc fragment of IgG. Columns were sterile, disposable devices that remove approx. one gram of IgG from plasma per treatment. As part of a multicenter clin. trial, 8 of 22 (36%) end stage breast cancer patients exhibited objective antitumor responses. Treatment of 24 patients with immune thrombocytopenia resulted in 14 (58) of patients having objective increases in platelet counts and clearance or significant reduction of anti-platelet antibody. The mechanism of action of this device is unknown at present but may be selective removal of IgG and CIC, and activation of factors such as the complement system which results in enhanced reticuloendothelial system (RES) clearance of CIC. In addition, immune modulation may result due to the stimulation of anti-idiotypic antibodies and selective clonal deletion of auto-antibody producing lymphocyte sub-populations.