Functional outcomes because of production of traditional and catalytic autoantibody. (A) The traditional IgG autoantibody binds 2 molecules of the autoantigen reversibly (top), whereas the catalytic autoantibody turns over repeatedly and can cleave thousands of autoantigens molecules over its lifetime in the blood (bottom). The inactivation of several autoantigens by catalytic autoantibodies has been described. (B) Reversibly binding autoantibodies can sterically mask the cleavage site in a zymogen, inhibiting its fragmentation by the zymogen-activating enzyme (top). In contrast, the catalytic autoantibodies described by Wootla et al themselves serve as the FIX zymogen-activating enzymes, resulting in the generation of the enzymatically active FIX fragment that in turn catalyzes FX activation (bottom).