Multifunctional cytokine secreted mainly by T-helper 2 lymphocytes and also mast cells or NKT cells that plays important roles in the immune response against parasites (PubMed:11070175, PubMed:19433802). Affects intestinal epithelial permeability and adaptive immunity (PubMed:12704113). In addition, induces the differentiation of specific T-cell subsets such as IL-17 producing helper T-cells (TH17) and also proliferation and differentiation of mast cells (PubMed:19433802, PubMed:11070175). Mechanistically, exerts its biological effects through a receptor composed of IL9R subunit and a signal transducing subunit IL2RG (PubMed:2145361, PubMed:7718508). Receptor stimulation results in the rapid activation of JAK1 and JAK3 kinase activities leading to STAT1, STAT3 and STAT5-mediated transcriptional programs (PubMed:10464327). Induction of differentiation genes seems to be mediated by STAT1 alone, while protection of cells from apoptosis depends on STAT3 and STAT5 (PubMed:10464327). Interacts with IL9R (PubMed:2145361). Interacts with IL2RG (PubMed:7718508). Deltion mice do not show defects in T-cell development or differentiation, the generation of naive or antigen-driven antibody responses, or the expulsion of the intestinal parasitic nematode Nippostrongylus brasiliensis. However, mastocytosis is severely impaired in these animals. Belongs to the IL-7/IL-9 family.