Like other investigators, we observed that GF mice have lower baseline levels of IL-4 compared to CONV-R mice.[56] Similarly, since the IL-23, IL-22, and IL-18 axis is involved in host defense and in mediating and regulating inflammatory immune responses upon parasitic and bacterial infection,[57–60] it is conceivable that GF mice would have lower circulating levels of IL-18 at baseline. Here, IL18 is linked to bacterial infectious disease.