Some nutrients are complex molecules (for example vitamin B) which would be destroyed if they were broken down into their functional groups.  To digest vitamin B non-destructively, haptocorrin in saliva strongly binds and protects the B molecules from stomach acid as they enter the stomach and are cleaved from their protein complexes.
After the B-haptocorrin complexes pass from the stomach via the pylorus to the duodenum, pancreatic proteases cleave haptocorrin from the B molecules which rebind to intrinsic factor (IF).  These B-IF complexes travel to the ileum portion of the small intestine where cubilin receptors enable assimilation and circulation of B-IF complexes in the blood.