Cluster Of Differentiation 8a (CD8a) is a cell surface glycoprotein predominantly found on cytotoxic T cells and natural killer cells. It serves as a coreceptor for the T cell receptor, binding to MHC class I molecules to enhance antigen recognition and T cell activation. Structurally, CD8α can form heterodimers with CD8β or homodimers, with the heterodimeric form being most common on cytotoxic T cells. This molecule plays a vital role in adaptive immunity by facilitating the elimination of virus-infected and cancerous cells. Beyond its effector functions, CD8α is essential for T cell development in the thymus.Besides,ACluster Of Differentiation 4 (CD4) has been identified as an interactor of CD8a, thus a functional binding ELISA assay was conducted to detect the interaction of recombinant rat CD8a and recombinant simian CD4 . Briefly, biotin-linked CD8a were diluted serially in PBS, with 0.01% BSA (pH 7.4). Duplicate samples of 100μl were then transferred to CD4-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 recombinant rat CD8a and recombinant simian CD4 was shown in Figure 1, the EC50 for this effect is 0.65µg/mL.