Intrinsic factor is a glycoprotein (produced by the
parietal cells of the stomach) that is required for the absorption of
vitamin B12 from the diet.1 During digestion, stomach acids dissociate B12
from food and intrinsic factor binds to it and allows it to be absorbed
in the small intestine. Conditions that impair intrinsic factor
production lead to B12 malabsorption and deficiency. Laboratory findings for B12 deficiency include decreased serum B12 levels, increased methylmalonic acid and megaloblastic anemia.2-6 Impaired hemoglobin synthesis associated with B12
deficiency is characterized by abnormal maturation of erythrocyte
precursors in the bone marrow, which results in the presence of
megaloblasts with hypersegmented neutrophils and decreased erythrocyte
survival.4 Vitamin B12 deficiency is also associated with neurological abnormalities.5,6
A leading cause of vitamin B12 deficiency is pernicious anemia (PA) caused by intrinsic factor deficiency.7-10
The condition is referred to as "pernicious" because it is clinically
silent initially and only becomes manifest when patients experience
generalized symptoms, such as weakness, diminished energy and (less
commonly) dyspepsia.9,11 The incidence of PA increases with age and is relatively rare in individuals younger than 30 years of age.10 The highest prevalence is seen in Northern Europeans, although PA has been reported in virtually every ethnic group.10
PA can be caused by pathologic conditions that damage or remove a
portion of the stomach's parietal cells, including bariatric surgery,
gastric tumors, gastric ulcers, and excessive consumption of alcohol.
Autoimmune ABG is caused by CD4 T cell-mediated autoimmune response
directed against the gastric H/K-ATPase.11 Diagnosis of autoimmune PA relies on histologically proven atrophic body gastritis, megaloblastic anemia, B12 deficiency, and antibodies to intrinsic factor and to gastric parietal cells.10
Antiparietal
cell antibodies are found in 90% of patients with PA, but have low
specificity and are seen in atrophic gastritis without megaloblastic
anemia as well as in various autoimmune disorders.10
Anti-intrinsic factor antibodies are less sensitive, being found in only
60% of patients with PA, but they are considered highly specific for
PA.9,11-13 Laboratory diagnosis is further supported by increased levels of fasting gastrin and decreased levels of pepsinogen I.7,9
Epidemiological
evidence and genetic studies suggest that PA has a significant
heritable component and leucocyte antigen-DR genotypes suggest a role
for genetic susceptibility.9,13,14 Long-standing Helicobacter pylori
infection may play a predisposing role in many patients in whom the
active infectious process has been gradually supplanted by an autoimmune
disease that terminates in a burned-out infection and the irreversible
destruction of the gastric body mucosa.9 PA is frequently
associated with autoimmune thyroid disease (40%) and other autoimmune
disorders, such as diabetes mellitus (10%), as part of the autoimmune
polyendocrine syndrome.8,9 PA incidence is also increased in patients with primary biliary cirrhosis compared to controls.15 Autoimmune gastritis may predispose to gastric carcinoid tumors or adenocarcinomas.8
