Dipeptidyl peptidase-4 (DPP4), also known as adenosine deaminase complexing protein 2 or cluster of differentiation 26 (CD26), is a protein in humans. DPP4 is an antigenic enzyme expressed on the surface of most cell types and is ass°Ciated with immune regulation, signal transduction and apoptosis. It is an intrinsic membrane glycoprotein and a serine exopeptidase that cleaves X-proline dipeptides from the N-terminus of polypeptides. Besides, Eosinophil Chemotactic Factor (ECF) has been identified as an interactor of DPP4, thus a binding ELISA assay was conducted to detect the interaction of recombinant human DPP4 and recombinant human ECF. Briefly, DPP4 were diluted serially in PBS, with 0.01% BSA (pH 7.4). Duplicate samples of 100uL were then transferred to ECF-coated microtiter wells and incubated for 2h at 37°C. Wells were washed with PBST and incubated for 1h with anti-DPP4 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 DPP4 and ECF was shown in Figure 1, and this effect was in a dose dependent manner.