Lastly, by use of mice with a simultaneous deficiency in the closely related cytokines IL-17A and IL-17F, we identified IL-17F as mediating the increased susceptibility in the absence of IL-17A after high-dose infection with Mtb H37rv, while it appeared to exert a minor impact on the enhanced infiltration of neutrophils in the IL-17A−/− mice. The gene discussed is IL17A; the disease is infection.