A 56-year-old woman ate contaminated fruit yesterday and developed fever, abdominal pain, and diarrhea at night. The stool contained pus and blood, was scanty, and occurred more than 10 times a day, accompanied by tenesmus. Physical examination: temperature 39.5°C, normal blood pressure (BP), clear consciousness, no abnormalities in heart or lungs, tenderness in the left lower abdomen without rebound tenderness, white blood cell (WBC) count 18×10<sup>9</sup>/L (N0.85), stool microscopy showing red blood cells (RBC) (++), and pus cells (+++). What is the most important pathogenesis of this type of bacillary dysentery?