Interleukin 5 Receptor Alpha (IL5Ra) is a transmembrane glycoprotein encoded by the IL5RA gene, serving as the specific subunit of the IL-5 receptor complex. Expressed primarily on eosinophils, basophils, and B lymphocytes, it mediates the biological effects of IL-5, a key cytokine in allergic inflammation and eosinophil homeostasis. IL5Ra forms a heterodimer with the common beta chain (βc) to constitute the functional receptor, enabling signal transduction via JAK/STAT pathways. Dysregulation of IL5Ra is linked to eosinophilic disorders, asthma, and allergic diseases, making it a therapeutic target for anti-allergy and anti-inflammatory drugs. IL5 binds specifically to IL5Ra with high affinity, inducing receptor dimerization and activating downstream signaling cascades that promote eosinophil survival and activation. 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.Measured by its binding ability in a functional ELISA. When Recombinant IL5Ra is lmmobilized at 2 µg/mL(100 uLwell), the concentration of IL5 that produces 50% optimal bindingresponse is found to be approximately 0.0930 µg/mL.