mAb 2D1 enhances the phagocytosis of 3D4/21 and RAW 264.6 cells to reference serovar 5 of G. parasuis.. After seeding 3D4/21 or RAW264.7 cells into a 12-well plate (approximately 4 × 105 cells/well), G. parasuis-5 (incubated with mAb 2D1 for 0.5 h at 37 °C) was added to cells at 200 MOI, and 5% (v/v) guinea pig serum was added to each cell well. After incubation at 37 °C in a 5% CO2 atmosphere for 2 h, complete growth medium (including 100 U/mL penicillin G and 0.25 mg/mL gentamicin) was added to each cell well. The cell plate was incubated for another 1 h to kill extracellular bacteria. At the same time, positive serum against G. parasuis-5, PBS and mAb ApxIV were used as positive and negative controls. Double-distilled water was used to lyse the cells, and lysates were evenly streaked onto TSA agarose plates. After growth overnight, colony numbers were measured (one-way ANOVA; Tukey’s post hoc test, ***p < 0.001, **p < 0.01, *p < 0.05, ns indicates no statistically significant difference).