Antibody titer upon heterologous challenge. Shown are humoral immune responses of control and vaccinated pigs on the day before and 21 days after infection, assessed using a detergent extract (deELISA), recombinant TbpB protein of A. pleuropneumoniae serotype 7 (TbpB-ELISA), recombinant ApxIV protein (ApxIV-ELISA), and the recombinant ApxIIA protein (ApxII-ELISA) as solid-phase antigen. The immune response was expressed in ELISA units (based on an external standard) for the standardized ApxII-ELISA, with activities of ≥25 ELISA units considered positive. Using the standardized ApxIV-ELISA, activities of ≥40% in comparison to an external control were considered positive; for the deELISA and the TbpB-ELISA, the immune response was expressed as a serum titer in comparison to that of an internal negative control.