C4BPa, also known as C4b-binding protein alpha chain or C4BPalpha, is a subunit of the complement factor C4b-binding protein (C4BP), which plays a crucial role in the regulation of the complement system, a part of the immune system that helps to defend the body against infection.C4BPa is part of a heterodimeric protein complex that consists of two chains, the alpha chain (C4BPa) and the beta chain (C4BPB). It can bind to C4b and C3b, thereby inhibiting the formation of the C3 convertase and preventing excessive complement activation. Thus a functional binding ELISA assay was conducted to detect the interaction of recombinant human C4BPa and recombinant mouse C4b. Briefly, C4BPa was diluted serially in PBS with 0.01% BSA (pH 7.4). Duplicate samples of 100 μl were then transferred to C4b-coated microtiter wells and incubated for 1h at 37℃. Wells were washed with PBST and incubated for 1h with anti-C4BPa 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 C4BPa and recombinant mouse C4b was shown in Figure 1, the EC50?for this effect is 0.01 ug/mL