IL9R (interleukin-9 receptor) is a cytokine receptor located in the pseudoautosomal region of the X and Y chrX|Y 1 that functions as a key mediator of IL-9 signaling in immune and stromal cells. As a plasma membrane receptor, IL9R binds IL-9 and activates JAK-STAT signaling pathways, particularly JAK1/JAK3-STAT1/STAT3/STAT4/STAT5, to regulate T cell differentiation, proliferation, and effector functions 234. IL9R plays critical roles in multiple disease contexts. In rheumatoid arthritis, IL9R-mediated signaling on CD4+ T cells sustains a pathogenic PU.1-IL9 positive feedback loop in Th9 cells that drives inflammatory activation 5. In acute myeloid leukemia, LSC-derived IL-9 signals through IL9R on CD4+ T cells to promote Th1 skewing and IFN-γ production, paradoxically enhancing LSC expansion 6. In atopic dermatitis, IL-9 signaling through IL9R on Th2 cells enhances IL-18-induced IL-13 and IL-22 secretion 2. Notably, IL9R+ CD8+ cytotoxic T cells characterize atopic dermatitis but are absent in chrX|Y nodular prurigo 7. In pregnancy, decreased placental IL9/IL9R expression impairs trophoblast proliferation and angiogenesis in preeclampsia 8. IL9R engineering in CAR T cells enhances anti-tumor efficacy through superior STAT signaling and memory differentiation 34.