Cluster of Differentiation 33 (CD33), a 67-kDa glycoprotein expressed in several cellular components, including Golgi apparatus; external side of plasma membrane; and peroxisome. Involved in several processes, including negative regulation of cytokine production; negative regulation of monocyte activation; and positive regulation of protein tyrosine phosphatase activity. A functional binding ELISA assay was conducted to detect the interaction of recombinant human CD33 and recombinant mouse CD19. Briefly, CD33 was diluted serially in PBS with 0.01% BSA (pH 7.4). Duplicate samples of 100 μl were then transferred to CD19-coated microtiter wells and incubated for 1h at 37℃. Wells were washed with PBST and incubated for 1h with anti-CD33 pAb, then aspirated and washed 3 times. After incubation with HRP labelled secondary antibody for 1h at 37℃, wells were aspirated and washed 5 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 450/630 nm immediately. The binding activity of recombinant human CD33 and recombinant mouse CD19 was shown in Figure 1, the EC50 for this effect is 1.88 ug/mL.