It was first noticed in 1929 that giving beef alone to Pernicious Anemia patients did not improve the anemia, but when given with human gastric juice, it did. Giving gastric juice alone (without beef) didn't help either. In 1936, the scientist, W.B Castle called the substance in beef 'Extrinsic Factor' and the substance in gastric juice 'Intrinsic Factor'. They only work when given together. Today we know Extrinsic Factor by a different name, vitamin B12.
The integrity of the cell is protected by a membrane barrier that is selective for what it allows in or keeps out. Embedded in the cell membrane are specialized proteins that recognize very specific molecules to transport into or out of the cell. Vitamin B12 is absorbed through one of these specialized receptors.
B12 is the largest and most complex vitamin and that makes it vulnerable to be damaged by digestive enzymes. Two proteins are specially designed to protect B12 in the GI tract. Haptocorrin protects B12 in the stomach and Intrinsic Factor (IF) protects it on its long journey down the small intestine to the B12 receptors for absorption.