IL9 and parasitic infectious disease: As a pleiotropic cytokine secreted by T-helper (Th)-2, Th-17, regulatory T (Treg) cells, mast cells, and innate lymphoid cells, IL-9 has been primarily examined in relation to mediating immune responses to parasitic infections and in the pathogenesis of Th2-linked immunological pathologies, such as asthma [42,43].