Why such a funny name? Well, there's a story. About 150 years ago, a strange anemia was described and was associated with thinning (or atrophy) of the stomach lining. It was called Pernicious Anemia. These patients often didn't survive for more than 3 years after diagnosis, but a life saving treatment diet of raw liver which contained an 'Extrinsic Factor' earned a Nobel Prize in 1934.
A scientist then figured out that the 'Extrinsic Factor' (B12) had to be working with an 'Intrinsic Factor' made in the stomach to cure Pernicious anemia.
And so it was. Intrinsic Factor, (otherwise known as IF) is a protein made in the stomach that is essential for ensuring the natural absorption of vitamin B12.
IF is needed to bind to B12, thereby protecting it from being damaged by acid and enzymes in the GI tract and taking it down the full length of the small intestine where special absorption receptors recognize the B12-IF complex, and only the complex, not B12 by itself, and not IF by itself.