ABSTRACTBackground.Helicobacter pyloriinfection persists in the presence of potent serum and gastric mucosal anti‐body responses against bacterial antigens. The aim of this article is to report on a study determine whether there is antibody deposition onH. pyloriin vivo in the stomach of infected patients and whether gastric and cultured forms ofH. pyloridiffer in their antibody reactivity.Materials and Methods.Serum, gastric biopsies, and antral brushings were obtained from 10 patients having endoscopy.H. pyloriwas cultured from gastric biopsies. Bacterial samples were stained directly for immunoglobulin deposition and indirectly using rabbit antiurease serum or patient serum. Samples were examined by immunofluorescence microscopy and flow cytometry.Results.Although spiral bacteria could be identified easily by acridine orange staining and antiurease staining of gastric brushings fromH. pyloriinfected patients, gastric bacteria did not have detectable IgG or IgA present, and only one of five samples could be stained for IgG and IgA indirectly using patient serum. In contrast, cultured bacteria could be stained readily with homologous serum for IgG and IgA in the majority of cases. Low pH inhibited immunoglobulin reactivity with culturedH. pylori.Conclusions.GastricH. pylorimay evade humoral defense owing to poor deposition of immunoglobulin in the gastric environment or failure to express surface antigens that are present on cultured forms ofH. pylor