© Springer Science+Business Media LLC 2017. Th9 cells are a new subpopulation of CD4+ T helper cells, characterized by the expression of IL-9 that have been involved in type 2 immune responses, antitumor responses and autoimmune diseases. Here, we describe two different parasitic models frequently maintained in the laboratory where Th9 cells or IL-9 (the cytokine produced by Th9 cells) has been shown to play critical roles in pathogen clearance and immune response activation: the nematode Nippostrongylus brasiliensis and the trematode Schistosoma mansoni.