AIM: To study the immunological protective effect of H pylori vaccine with chitosan as an adjuvant and its mechanism.METHODS: Female BALB/c mice were randomly divided into seven groups and orally immunized respectively with PBS, chitosan solution, chitosan particles, H pylori antigen, H pylori antigen plus cholera toxin (CT), H pylori antigen plus chitosan solution, H pylori antigen plus chitosan particles once a week for four weeks. Four weeks after the last immunization, the mice were challenged twice by alive H pylori (1 × 109 CFU/mL) and sacrificed. Part of the gastric mucosa was embedded in paraffin, cut into sections and assayed with Giemsa staining. Part of the gastric mucosa was used to quantitatively culture H pylori. ELISA was used to detect cytokine level in gastric mucosa and anti- H pylori IgGl, IgG2a levels in serum.RESULTS: In the groups with chitosan as an adjuvant, immunological protection was achieved in 60% mice, which was significantly higher than in groups with H pylori antigen alone and without H pylori antigen (P ＜ 0.05 or 0.001). Before challenge, the level of IFN and IL-12 in gastric mucosa was significantly higher in the groups with chitosan as an adjuvant than in the control group and the group without adjuvant (P ＜ 0.05 or 0.005). After challenge, the level of IFN and IL-12 was significantly higher in the groups with adjuvant than in the groups without adjuvant and antigen (P ＜ 0.05 or 0.001). Before challenge, the level of IL-2 in gastric mucosa was not different among different groups. Afterchallenge the level of IL-2 was significantly higher in the groups with adjuvant than in the control group (P ＜ 0.05 or 0.001). Before challenge, the level of IL-10 in gastric mucosa was significantly higher in the groups with chitosan as an adjuvant than in other groups without adjuvant (P ＜ 0.05 or 0.01). After challenge, the level of IL-10 was not different among different groups. Before challenge, the level of IL-4 in gastric mucosa was significantly higher in the groups with chitosan as an adjuvant than in other groups without adjuvant (P ＜ 0.05). After challenge, the level of IL-4 was significantly higher in the groups with chitosan particles as an adjuvant than in the group with CT as an adjuvant (P ＜ 0.05), and in the group with chitosan solution as an adjuvant, the level of IL-4 was significantly higher than that in control group, non-adjuvant group and the groups with CT (P ＜ 0.05 or 0.001). The ratio of anti- H pylori IgG2a/IgGl in serum was significantly lower in the groups with chitosan as an adjuvant than in the groups with CT as an adjuvant or without adjuvant (P ＜ 0.01).CONCLUSION: H pylori vaccine with chitosan as an adjuvant can protect against H pylori infection and induce both Thl and Th2 type immune response.