Interleukin 5 (IL5) is a T cell-derived cytokine crucial for eosinophil growth, differentiation, and activation, playing key roles in allergic inflammation and parasite defense. This homodimeric glycoprotein signals through the interleukin 5 receptor complex, composed of IL5Ra (CD125) and common beta chain (CD131). IL5 specifically binds IL5Ra with high affinity , initiating JAK/STAT signaling pathways that promote eosinophil survival and function.To detect the activity of recombinant IL5, a functional ELISA assay was performed to evaluate the interaction between recombinant human IL5 and recombinant human IL5.Briefly, IL5 was diluted serially in PBS with 0.01% BSA (pH 7.4). Duplicate samples of 100 μl were then transferred to IL5Ra-coated microtiter wells and incubated for 1h at 37℃. Wells were washed with PBST and incubated for 1h with anti-IL5 pAb, then aspirated and washed 3 times. After incubation with HRP labelled secondary antibody for 1h at 37℃, wells were aspirated and washed 5 times. With the addition of substrate solution, wells were incubated 15-25 minutes at 37℃. Finally, add 50 μL stop solution to the wells and read at 450/630nm immediately. The binding activity of recombinant human IL5 and recombinant human IL5Ra was shown in Figure 1, the EC50?for this effect is 0.047μg/mL.