Cluster of Differentiation 2 (CD2) is a surface antigen found on all peripheral blood T-cells. CD2 functions as an activator of T lymphocytes and thymocytes. Mature human CD2 consists of a 185 amino acid (aa) extracellular domain (ECD) with an Ig-like V-type and an Ig-like C2 type domain, a 26 aa transmembrane segment, and a 116 aa cytoplasmic tail. Cluster of Differentiation 5 (CD5) has been identified as an interactor of CD2, thus a functional binding ELISA assay was conducted to detect the interaction of recombinant human CD2 and recombinant human CD5. Briefly, CD2 were diluted serially in PBS, with 0.01% BSA (pH 7.4). Duplicate samples of 100 µl were then transferred to CD5-coated microtiter wells and incubated for 2h at 37℃. Wells were washed with PBST and incubated for 1h with anti-CD2 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 50 µL stop solution to the wells and read at 450 nm immediately. The binding activity of recombinant human CD2 and recombinant human CD5 was shown in Figure 1, the EC50 was 1.88 ug/ml.