Intrinsic factor. Intrinsic factor. a mucoprotein normally secreted by the epithelium of the stomach and that binds vitamin b12, the intrinsic factor/B12 complex is selectively absorbed by the distal ileum, though only the vitamin is taken into the cell.ntrinsic factor. Intrinsic factor. a mucoprotein normally secreted by the epithelium of the stomach and that binds vitamin b12, the intrinsic factor/B12 complex is selectively absorbed by the distal ileum, though only the vitamin is taken into the cell.