Studies on the specific role of ICOS in immunity to bacterial, viral or parasite infection have been greatly facilitated by the availability of mice lacking ICOS or using systemic delivery of blocking antibodies and revealed that the absence or blockade of ICOS leads to either unaffected of reduced CD4+ or CD8+ T cell responses [5], [14], [27]–[30]. This evidence concerns the gene ICOS and parasitic infectious disease.