C4B and neoplasm: These data suggest that binding of C1q to the Fc part of CD19-DEK is not sufficient to block FcR engagement and that probably deposition of other complement factors such as C3b/c and C4b/c on the target cell surface may play a dominant role in the reduction of tumor cell lysis mediated by CD19-DEK in whole blood assays supplemented with eculizumab.