Interleukin 9, also known as IL-9, is a pleiotropic cytokine belonging to the group of interleukins. IL-9 secreted by CD4 helper cells that acts as a regulator of a variety of hematopoietic cells. This cytokine stimulates cell proliferation and prevents apoptosis. It functions through the interleukin-9 receptor (IL9R), which activates different signal transducer and activator (STAT) proteins namely STAT1, STAT3 and STAT5 and thus connects this cytokine to various biological pr°Cesses. Besides, Interleukin 9 Receptor (IL9R) has been identified as an interactor of IL-9, thus a binding ELISA assay was conducted to detect the interaction of recombinant human IL-9 and recombinant human IL9R. Briefly, IL-9 were diluted serially in PBS, with 0.01% BSA (pH 7.4). Duplicate samples of 100µL were then transferred to IL9R-coated microtiter wells and incubated for 2h at 37°C. Wells were washed with PBST and incubated for 1h with anti-IL-9 pAb, then aspirated and washed 3 times. After incubation with HRP labelled secondary antibody, wells were aspirated and washed 3 times. With the addition of substrate solution, wells were incubated 15-25 minutes at 37°C. Finally, add 50µL stop solution to the wells and read at 450nm immediately. The binding activity of IL-9 and IL9R was shown in Figure 1, and this effect was in a dose dependent manner.