2.2.1. Synthesis of Antigens MQCA–PABA–BSA/OVA

The synthesis protocol of MQCA–PABA–BSA/OVA is shown in Figure 2A. Briefly, MQCA (90.0 mg), dicyclohexylcarbodiimide (DCC, 110.0 mg), and N-hydroxysuccinimide (NHS, 55.0 mg) were dissolved with 1.2 mL of N, N-dimethylformamide (DMF), and the mixture was stirred at 4 ◦C overnight. After removing the precipitate by centrifugation at 6000× *g*, the supernatant was obtained, PABA (68.0 mg) was added, and the reaction was continued for 24 h with stirring. After centrifugation, the supernatant was collected and the solution became turbid after adding distilled water (10 mL). The precipitate obtained by filtering through filter paper is MQCA–PABA.

**Figure 2.** The synthesis protocol of MQCA–PABA–BSA/OVA (**A**) and DMEQ–AOAA–HSA/OVA (**B**).

Then, MQCA–PABA (30.0 mg), NHS (15.0 mg), and DCC (20.2 mg) were dissolved with 1.0 mL of DMF, and the mixture was stirred at 4 ◦C overnight. After centrifugation, the supernatant was added slowly with precipitate to the solution of 15 mL phosphate-buffered saline (PBS, 0.01 mol/L, pH 7.4) containing 66.0 mg BSA. The mixture was stirred for 24 h at 4 ◦C and then dialyzed in PBS for 5 d, and the dialysate was changed every 8 h. The dialyzed sample (MQCA–PABA–BSA) was collected and stored at −20 ◦C until use. The same principle can be followed to obtain MQCA–PABA–OVA.
