Complement Component 1, Q Subcomponent A (C1qA) is a protein that plays a crucial role in the innate immune system. It is part of the C1 complex, which is the first component of the classical complement pathway. The C1 complex is composed of one molecule each of C1q, C1r, and C1s. C1q is the recognition subcomponent of the complex and and initiating the complement cascade. C1qA plays a key role in eliminating pathogens, regulating inflammatory response and maintaining immune tolerance. It is reported that C1qA can bind to the C-terminal of CALR through its gelatin-as-region, and this interaction may play an important role in regulating complement activation and cell signaling. Thus a functional binding ELISA assay was conducted to detect the interaction of recombinant mouse C1qA and recombinant human CALR. Briefly, C1qA was diluted serially in PBS with 0.01% BSA (pH 7.4). Duplicate samples of 100 μl were then transferred to CALR-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 450/630 nm immediately. The binding activity of recombinant mouse C1qA and recombinant human CALR was shown in Figure 1, the EC50?for this effect is 0.03 ug/mL.