Gregg J. Silverman, Carl S. Goodyear, and Don L. Siegel ith the development of online blood cell separators in the early 1950s and the subsequent appearance of automated microprocessor-controlled apheresis devices, therapeutic plasma exchange has become an important therapeutic modality for the treatment of a wide range of autoimmune, hematologic, neurologic, and metabolic disorders. For the removal of pathogenic substances, however, conventional plasmapheresis can be an inherently inefficient process that typically removes significant quantities of “healthy” plasma constituents in parallel with the depletion of the presumed disease-causing material (eg, autoantibodies), which usually comprises only a very small fraction of total plasma protein. This has led to the development of a number of selective extraction approaches in which patient plasma is perfused over filters or affinity columns containing immobilized ligands that display a certain degree of specificity for the offending substance (reviewed in Vamvakas and Pineda1). In the US, one of the first such devices to receive FDA approval was the Prosorba column (currently marketed by Fresenius HemoCare, Inc., Redmond, WA), which comprises 200 mg of staphylococcal protein A (SpA) covalently coupled to an inert silica matrix. For the past two decades, this device has been used to treat a wide variety of diseases