Cluster Of Differentiation 55 (CD55), also known as Decay Accelerating Factor (DAF), is a glycosylphosphatidylinositol (GPI)-anchored membrane protein that belongs to the RCA (regulators of complement activation) family and is expressed on all cells that are bathed in plasma, such as blood cells and endothelial cells. CD55 functions as a complement regulator, accelerating the decay of C3 and C5 convertases, thereby inhibiting the formation of the membrane attack complex (MAC) and preventing complement-mediated cell lysis. It plays a crucial role in protecting host cells from complement damage and is involved in various physiological and pathological processes, including immune complex clearance and inflammation.When CD55 binds to C4a, CD55 can regulate the activity of the complement system and prevent cell damage caused by over-activation of the complement.. Thus a functional binding ELISA assay was conducted to detect the interaction of recombinant human CD55 and recombinant human C4a. Briefly,CD55 was diluted serially in PBS with 0.01% BSA (pH 7.4). Duplicate samples of 100 μl were then transferred to C4a-coated microtiter wells and incubated for 1h at 37℃. Wells were washed with PBST and incubated for 1h with anti-CD55 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 450/630nm immediately. The binding activity of recombinant human CD55 and recombinant human C4a was shown in Figure 1, the EC50 for this effect is0.18ug/mL.