Given our observations of increased ileal mastocytosis and MC activation in baso IL-4/IL-13 (−) mice (Fig. 1), along with our prior observations of MC-dependent intestinal permeability and bacterial translocation in P. yoelii infection (7, 8), we sought to assess the effects of basophil-derived IL-4 and IL-13 on intestinal permeability and bacteremia in P. y. yoelii 17XNL–infected mice. This evidence concerns the gene IL4 and mastocytosis.