a glycoprotein (i.e., a complex compound containing both polysaccharide and protein components) with which vitamin B₁₂ (cyanocobalamin (vitamin B12)) must combine to be absorbed by the gut. Intrinsic factor is secreted by parietal cells of the gastric glands in the stomach, where it binds with the vitamin. Thus bound, intrinsic factor protects vitamin B₁₂ from digestion as it passes through the gastrointestinal tract and facilitates the vitamin's absorption in the ileum of the small intestine. Vitamin B₁₂ is necessary for red blood cell maturation. A lack of intrinsic factor may result in inadequate absorption of the vitamin and cause pernicious anemia.

The term intrinsic factor was coined in the late 1920s by the American physician William B. Castle, whose research into the cause of pernicious anemia indicated that two substances were involved: one that is produced in the body (intrinsic) and the other—an extrinsic factor, later identified as vitamin B₁₂—that is supplied in the diet.