Mechanisms that restrict the immune response are critical to avoid damaging reactivity that can lead to allergies and inflammatory responses or, in more severe cases, to lethal autoimmune diseases. Our goal is to understand mechanisms that downregulate the immune response, control lymphocyte proliferation and antibody production. With this intention we initiated a study of the negative regulatory pathway mediated by the inositol phosphatase SHIP. Absence of SHIP results in reduced viability of the mice due to a myeloproliferative-like syndrome, with profound splenomegaly and massive myeloid cell accumulation in the lungs. The very pleiotropic phenotype of SHIP-/- mice reflects the fact that SHIP is a general down-modulator of growth factor, cytokine, Fc and antigen receptor activity. To get a more precise idea of the in vivo function of SHIP in specific cells while avoiding pleiotropic interactions, we have begun to characterize tissue-specific or inducible SHIP mutations in mice. We have generated loxP-flanked SHIP mice that can be crossed to several Cre recombinase-expressing mice. We have characterized the functional role of SHIP in T lymphocyte responses of the Th2 type, which are important in parasite elimination. In collaboration with Tom Wynn, and confirming the role of SHIP in Th2 responses in vivo, we have determined that SHIP is required in T cells for an efficient response agains Schistoma mansoni.