IL21 and bacterial infectious disease: IL-1ß, IL-6, and IL-21 have been shown to initiate the differentiation of T cells toward the Th17 lineage, and the main effector cytokines of Th17 cells are IL-17 and IL-22.30–32 Th17 cells became the focus of attention as they play a protective role during bacterial infections; however, they seem to mediate tissue damage.33–36 A correlation of pathologies such as hepatitis with high IL-17 levels in the serum of O. tsutsugamushi–infected patients could not be found by others,37 so it is likely that the Th17-type response is rather beneficial for the host than detrimental.