Pernicious anemia is the end stage of an autoimmune gastritis resulting in destruction of the gastric mucosa. Only a small proportion (10-15%) of patients with autoimmune gastritis develop pernicious anemia. Nonetheless, at an estimated prevalence of 1.9% in Western adult populations at or over the age of 60 years, pernicious anemia represents the commonest cause of vitamin B12 deficiency in these populations.
Although "silent" until the end stage, the gastric lesion can be predicted years before clinical presentation by immunologic markers specific for gastric autoimmunity. The gastritis results in a deficiency of intrinsic factor, a protein which binds avidly to dietary vitamin B12 and promotes its transport to the terminal ileum for absorption. Consequently, the gastritis is expressed clinically as vitamin B12 deficiency associated with megaloblastic anemia. The anemia was indeed "pernicious" when first discovered but is now readily controlled by vitamin B12 treatment.