Previous studies have demonstrated that in vivo IL-33 treatment results in significant infiltration of eosinophils, myeloid cells, and plasma cells within the spleen.15,35 Consistent with these reports, after 6 consecutive days of IL-33 treatment (1 μg/d, harvested on day 7), we observed significant splenomegaly and a nearly 2-fold increase in splenocyte cell numbers in IL-33–treated mice compared with saline-treated control mice (P = .0012, Figure 1A). Here, IL33 is linked to Splenomegaly.