Helicobacter pyloriinfection has been recognized as the causative agent of type B chronic active gastritis. It also shows a close correlation with duodenal and gastric ulcer disease. Its role in the pathogenesis of gastric malignancy is disputed. Characteristically, an intense local immune response may be seen on histologic examination of gastric mucosa. So far, little is known about immunologic reactions within the stomach, and only recently has it been appreciated that the gastric mucosal immune response may represent one main pathogenetic mechanism inH. pylori-associated diseases. This review gives a brief background on what is known about immunophysiologic mechanisms in the stomach and relates these mechanisms to recent studies evaluating the immune response toH. pylori.