Internal factor Antibodies

General Information

Internal factor - glycoprotein (a complex protein that contains both a polysaccharide or carbohydrate component and a protein component), which is synthesized by the parietal cells of the stomach and is a necessary component for the absorption of vitamin B12 (it also participates in the absorption of other B substances).

The term "intrinsic factor" was named by the American doctor William Castle, who discovered that 2 factors are involved in the formation of pernicious anemia: internal - which is produced by the parietal cells of the stomach, and external - which was later called vitamin B12.

Autoantibodies against intrinsic factor (FI) are characteristic of pernicious anemia and are found in the blood serum of 56% of patients. These antibodies are of class G and A. Only IgG is present in serum (IgA may be present in gastric juice).

Autoantibodies prevent binding of vitamin B12 to the intrinsic factor, or bind to already formed complexes (intrinsic factor + vitamin B12) and prevent its absorption.

Autoantibodies against intrinsic factor (FI) are found in a large percentage of children with juvenile pernicious anemia.

When should we do the research? –To confirm the diagnostics of pernicious anemia

Preparation of the patient: Vitamin B2 intake should be stopped for 12 weeks before the analysis

Material for examination: Venous blood

Referral norm:   <6 units/mL: negative

Interpretation of results

Determination of autoantibodies against intrinsic factor (FI) is important for differential diagnostics of vitamin B12 deficiency. Megaloblastic anemia associated with low serum vitamin B12 levels and anti-intrinsic factor (FI) autoantibodies is evidence of pernicious anemia. At the same time, a negative response to antibodies does not exclude the diagnostics of pernicious anemia, since 50% of patients do not develop antibodies at all.

Limitation and Contradiction

Determination of autoantibodies against intrinsic factor (FI) is not recommended in patients who have been treated with injectable vitamin B12 for at least 2 weeks. Elevated levels of free vitamin B12 in the serum may be observed within 2 weeks of injection treatment, which may give a false positive response in the anti-intrinsic factor (FI) autoantibodies test. Thus, a positive result obtained in the determination of autoantibodies against intrinsic factor (FI) requires also the determination of serum vitamin B12 concentration.

In patients with autoimmune diseases, autoantibodies against intrinsic factor (FI) may be positive even without the presence of pernicious anemia.