Theαandβsubunits of the gastric H+/K+-ATPase (proton pump) have been identified as the major molecular targets of parietal cell autoantibodies associated with pernicious anaemia and with murine experimental autoimmune gastritis (BAG) induced by neonatal thymectomy. Recent studies with BAG suggest that the mechanisms of peripheral tolerance and autoimmunity to extrathymic autoantigens are mediated by subsets of“regulator”and“effector”CD4+T cells, respectively. The persistence of“effector”CD4+autoreactive T cells in the periphery may be a direct consequence of the delayed developmental expression of the target autoantigen. We hypothesize that cytokines produced by the“regulator”T cells prevent the clonal expansion of the“effector”autoreactive T cells, and that neonatal thymectomy induces organ-specific autoimmunity in genetically susceptible individuals by the reduction of the“regulator”T cell population.