F9 and acquired hemophilia: However, the description of catalytic IgM, IgG and IgA under physiological conditions [13], [14], [15], the finding that levels of PFR-MCA hydrolyzing IgG were elevated in patients surviving from septic shock [21], and the discovery in some patients with acquired hemophilia, a disease characterized by the presence of neutralizing anti-factor VIII autoantibodies, of factor IX-activating IgG [32], encouraged to revisit the initial hypothesis.