Unlike anti-PT IgG, which was specifically produced after contact with Bordetella pertussis, other infections such as respiratory pathogens, Bordetella parapertussis and Haemophilus influenzae caused cross-antigen reactions that also inducing the production of anti-FHA IgG (26–29), and the kinetic studies had revealed a slower decline of anti-FHA IgG than anti-PT IgG (30). This evidence concerns the gene F2 and infection.