Complement Component 1, Q Subcomponent A (C1qA) is a subunit of the C1q complex in the complement system. It has a crucial role in recognizing pathogens and immune complexes. Structurally, it contributes to the overall architecture of C1q, enabling it to interact with various ligands. Functionally, it is essential for initiating the classical complement pathway.Binding of C1qA to C1r activates C1r, triggering the classical complement cascade for pathogen elimination and immune response regulation.Thus a functional ELISA assay was conducted to detect the interaction of recombinant human C1qA and recombinant mouse C1r. Briefly, C1qA was diluted serially in PBS with 0.01% BSA (pH 7.4). Duplicate samples of 100 μl were then transferred to C1r-coated microtiter wells and incubated for 1h at 37℃. Wells were washed with PBST and incubated for 1h with anti-C1qA 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 C1qA and recombinant mouse C1r was shown in Figure 1, the EC50 for this effect is 0.022ug/mL.