Protein drugs like insulin are damaged in the gastrointestinal tract and cannot be given orally and retain efficacy. Protease enzymes like trypsin and chymotrypsin are designed to break down proteins and prepare them for absorption.
IF binds to B12 and protects it from being destroyed by trypsin and chymotrypsin in the intestine. That is the reason IF is designed to be resistant to these enzymes. 
IF protects B12 and a published in-vitro study has shown that IF may protect protein drugs that are attached to B12 as well. This could be a 'game-changer' for certain drugs.
An in-vitro study also showed that IF has the potential to protect protein drugs in the circulation against a powerful protein degrading enzyme called meprin, which could extend the activity of injected protein drugs. 
IF, which is shaped like a donut, pulls most of the B12 molecule into the 'donut hole' and this protects the more vulnerable parts of the B12 molecule from destruction on its long journey (20 feet) to the receptors. 
A receptor is a protein that straddles a cell membrane and is designed to recognize and transport specific molecules across the membrane into the cell. 
Vitamin B12 by itself is not recognized by the B12 receptor. It needs to be attached to IF. The IF-B12 complex is the 'key' to the lock that absorbs B12. 
Conjugating (joining) B12 to a protein drug allows the IF to become associated with the drug and creating a 3-member complex that could be recognized by B12-IF receptors and potentially gain access to the body via the oral route. [4-8]
Xeragenx's IF (rh-IF) has also been shown in a pre-clinical study to be recognized by specific multi-ligand endocytic mannose receptors (CD206) of activated macrophages and can be used to target specific organs like liver and lung.[9,10]