ADAMTS13 and thrombotic thrombocytopenic purpura: To directly test whether the IgG subclass of anti-ADAMTS13 autoantibodies has any impact on their pathogenicity, a pathogenic monoclonal anti-ADAMTS13 antibody (clone TTP1-420) previously isolated from TTP patients and confirmed in mice (Ostertag et al., 2016a; Ostertag et al., 2016b) was expressed as either human IgG4 or IgG1 antibodies.