Platelet/Endothelial Cell Adhesion Molecule (PECAM1), also known as CD31, is a transmembrane glycoprotein expressed on endothelial cells, platelets, and leukocytes. It has six immunoglobulin - like domains.PECAM1 maintains vascular integrity via homophilic binding, regulating endothelial cell migration during angiogenesis. It also modulates immune responses by influencing leukocyte movement across the endothelium.Notably, its interaction with Integrin Beta 2 (CD18) promotes leukocyte - endothelial adhesion and transmigration, key for immune cell recruitment to inflammation sites.Thus a functional binding ELISA assay was conducted to detect the interaction of recombinant rat PECAM1 and recombinant human CD18 . Briefly, biotin-linked PECAM1 were diluted serially in PBS, with 0.01% BSA (pH 7.4). Duplicate samples of 100μl were then transferred to CD18-coated microtiter wells and incubated for 1h at 37℃. Wells were washed with PBST 3 times and incubation with Streptavidin-HRP for 30min, then wells were aspirated and washed 5 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 PECAM1 and CD18 was shown in Figure 1,and this effect was in a dose dependent manner.