SERPINA3 and infection: Demonstration that ACT binds to FhaB on the surface of B. pertussis and B. bronchiseptica (43), together with the revelation that full-length FhaB plays an important role during infection (11) and that the FhaB prodomain is degraded in a regulated manner (24, 25), suggested a model in which the binding of ACT to its receptor on a phagocytic cell triggers degradation of the FhaB prodomain and delivery of ACT to the host cell (Fig. 1).