Several lines of evidence implicateHelicobacter pylori (H. pylori) infection in gastroduodenal inflammation. However, the exact pathogenesis ofH. pylori infection is not fully understood. T-helper (TH) lymphocytes may be subdivided into TH1 and TH2 cells based on the distinct patterns of cytokine production. TH1 reaction is associated with immunity or resistance to infection, while TH2 reaction is associated with the progression or persistence of infection. The production of interferon-γ (INF-γ) and interleukin 2 (IL-2), which are type 1 cytokines, is decreased inH. pylori infection. Enhanced production of type 2 cytokines (IL-4) and (IL-6) is observed in individuals withH. pylori infection. Suppressed proliferative responses of peripheral blood and gastric lymphocytes have also been demonstrated in patients withH. pylori colonisation, suggesting that specific T-cell responses may be down-regulated by an enhanced TH2 reaction. Suppressed TH1 and enhanced TH2 responses inH. pylori infection may be involved in the immunopathogenesis of chronicH. pylori infection.