IntroductionHelicobacter pylori infections induce vigorous systemic and mucosal humoral responses that are predominantly mediated by IgA, IgG, and IgM. These immunoglobulins are detectable in sera, gastric aspirates, or stomach extracts (1,2). Humoral immunity against H. pylori can effectively prevent infection and reduce colonization but does not lead to eradication of H. pylori-induced gastritis (2).Absorption of dietary cobalamin (vitamin B 12 ; vB 12 ) depends on several factors, including acid-dependent deproteinization of vB 12 . Only free vB12 can form a complex with the intrinsic factor (IF). The vB12-IF complex is then absorbed by mucosal cells via its specific receptor (cubilin) in a calcium-dependent fashion. Gastric parietal cells are responsible for production of both hydrochloric acid and IF. Absorbed vB12 is then stored in the liver. Some H. pylori patients develop autoantibodies directed against gastric parietal H + /K + -ATPase cells (APCAs), resulting in achlorhydria and increased infection with H. pylori, which in turn contributes to gastric damage and atrophy of the corpus (2).Autoimmunity may destroy parietal cells that are responsible for hydrochloric acid and IF production, resulting in achlorhydria and failure to deproteinize vB 12 and/or failure to produce IF. Consequently, the vB 12 -IF complex fails to form, leading to malabsorption of vB 12 and, potentially, to pernicious anemia. Hence, patients diagnosed with vB 12 deficiency as a result of APCAs or anti-IF antibodies were excluded, since their vB 12 deficiency cannot be directly and solely attributed to H. pylori infection (3-6; see Section 2.2).In this work, we investigated the correlation between serum anti-H. pylori IgA antibodies and vB 12 . The role of contaminating variables on this correlation was explored as well. Methods and materials PatientsBlood samples were collected in 5-mL tubes from 133 vB 12deficient patients (60 males, 73 females) and 105 healthy volunteers (42 males, 63 females), aged 18-50 years (mean, 34.1 years). All 133 patients had vB 12 deficiency and Background/aim: H. pylori infection and vitamin B 12 (vB 12 ) deficiency have high prevalence rates among Palestinians. It was observed that most people who suffered from vB 12 deficiency were positive for H. pylori. Materials and methods:The correlation between H. pylori infection and vB 12 deficiency was investigated in a representative segment of the Palestinian population. ELISA was used to determine levels of vitamin B 12 (vB 12 ) and anti-H. pylori IgA in sera from 238 participants from Al-Khalil district (Hebron), Palestine.Results: There was a strong negative Pearson's correlation coefficient (r = -0.45; P = 0.00001) between levels of anti-H. pylori IgA and vB 12 levels in sera drawn from 238 participants (133 patients and 105 control subjects). Two important contaminating variables were identified in this study: healthy control subjects with elevated anti-H. pylori IgA titers and vB 12 -deficient patients testing negative for anti-H. pylori IgA antibodi...