At 0, 24, and 144 h after ETEC challenge, the blood samples were centrifuged at 2,000 × g for 20 min at 4°C. The resultant sera were obtained and stored at −80°C. The levels of immunoglobulin A (IgA), immunoglobulin M (IgM), immunoglobulin G (IgG), D-lactate, and diamine oxidase (DAO) activity were then analyzed by using ELISA kits (Shanghai MLBIO Biotechnology Co. Ltd., China).