Given that basophils generate IL-4, IL-13, and histamine in response to IL-18 (17), and that IL-4, IL-13, and IL-18 are increased in plasma in our model (9), we used mice with basophils floxed for the adjacent genes Il4 and Il13 [baso IL-4/IL-13 (−) mice] and genotype controls to test the hypothesis that basophil-derived IL-4 and IL-13 control mastocytosis and malaria-induced intestinal permeability and bacteremia. The gene discussed is IL4; the disease is bacterial infectious disease with sepsis.