IL-9 + CD4+T cells comprise a heterogeneous population of T cells including T helper (Th)-2, 1 Th17, 2 regulatory T cells 3 and a recently discovered T-cell subset named Th9. 4 Closely related to Th2, Th9 is deviated from naïve CD4+ T cells in the presence of transforming growth factor (TGF)-β, interleukin (IL)-4, IL-21, tumour necrosis factor (TNF)-α and IL-10. 5-8 Although IL-9 is known as prototype cytokine of Th9, these cells cannot produce other cytokines such as IL-4, IL-5, IL-13 and interferon (IFN)-γ. 9 To produce IL-9, several cytokines (IL-1α/β, IL-2, IFN-α and IL-33) are considered as the activators, 5 and on the other hand, interferon (IFN)-γ is reported to play inhibitory effects on Th9 differentiation and IL-9 production both in vitro and in vivo. 10 Collectively, as shown in Figure 1, various cytokines are involved in the differentiation of IL-9 + CD4+T cells as well as IL-9 secretion. Although the role of IL-9 + CD4+T cells and its prominent cytokine, IL-9, has been clarified in the autoimmune diseases, 11 allergic diseases 12,13 and tumours, 14 limited studies have investigated its role in the infectious diseases. Leishmaniasis, as an infectious disease, is caused by obligatory intracellular protozoan parasite belonging to the genus Leishmania (L). Based on the Leishmania species, host genetics, interaction between host immune system and parasites and the load of Leishmania entrance, the clinical manifestations of the disease range from self-healing cutaneous leishmaniasis (CL) to life-threatening form of visceral leishmanisis (VL)