The gastric mucosa separates the underlying tissue from the vast array of antigens that traffic through the stomach lumen. While the extreme pH of this environ- ment is essential in aiding the activation of enzymes and food digestion, it also renders the gastric epithelium free from bacterial colonization, with the exception of one important human pathogen, H pylori. This bacterium has developed mechanisms to survive the harsh environment of the stomach, actively move through the mucosal layer, attach to the epithelium, evade immune responses, and achieve persistent colonization. While a hallmark of this infection is a marked inflammatory response with the in- filtration of various immune cells into the infected gastric mucosa, the host immune response is unable to clear the infection and may actually contribute to the associ- ated pathogenesis. Here, we review the host responses involved during infection with H pylori and how they are influenced by this bacterium. ? 2006 The WJG Press. All rights reserved. The gastric mucosa separates the underlying tissue from the vast array of antigens that traffic through the stomach lumen. While the extreme pH of this environ ment is essential in aiding the activation of enzymes and food digestion, it also renders the gastric epithelium free from bacterial colonization, with the exception of one important human pathogen, H pylori. This bacterium has developed mechanisms to survive the harsh environment of the stomach, actively move through the mucosal layer, attach to the epithelium, evade immune responses, and achieve persistent colonization. While a hallmark of this infection is a marked inflammatory response with the in-filtration of various immune cells into the infected gastric mucosa, the host immune response is unable to clear the infection and may actually contribute to the associ- ated pathogenesis. Here, we review the host responses involved during infection with H pylori and how they are influenced by this bacterium.? 2006 The WJG Press . All rights reserved.