Interleukin 3 receptor alpha (IL3Ra), also known as CD123 (Cluster of Differentiation 123), is a subunit of the functional high-affinity mouse IL-3 receptor which is a heterodimer. The alpha subunit alone binds IL-3 with low affinity. The beta subunit does not bind IL-3 by itself but is required for the high-affinity binding of IL-3 to the heterodimeric receptor complex. Both the alpha and the beta subunits are members of the cytokine receptor superfamily. A binding ELISA assay was conducted to detect the interaction of recombinant mouse Interleukin 3 Receptor Alpha and recombinant mouse Interleukin 3. Briefly, IL3Ra were diluted serially in PBS, with 0.01% BSA (pH 7.4). Duplicate samples of 100 µl were then transferred to IL3-coated microtiter wells and incubated for 2h at 37℃. Wells were washed with PBST and incubated for 1h with anti-IL3Ra 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℃. Finally, add 50µL stop solution to the wells and read at 450nm immediately. The binding activity of IL3Ra and IL3 was shown in Figure 1, the EC50 was 0.173 ug/ml.