T helper cell
The requirements of IL-17A and IL-17 receptor signaling in host defense were well documented and appreciated before the identification of Th17 cells as an independent T helper cell lineage. In experimental pneumonia models, IL-17A or IL-17RA knock mice have increased susceptibility to various Gram-negative bacteria, such as "Klebsiella pneumoniae" and "Mycoplasma pneumoniae". In contrast, data suggest that IL-23 and IL-17A are not required for protection against primary infection by the intracellular bacteria "Mycobacterium tuberculosis". Both the IL-17RA knock out mice and the IL-23p19 knock out mice cleared primary infection with "M. tuberculosis". However, IL-17A is required for protection against primary infection with a different intracellular bacteria, "Francisella tularensis".