B12 Absorption. 1. Dietary B12 binds to intrinsic factor (IF), secreted by gastric parietal cells. 2. The B12-IF complex then binds to the IF receptor on terminal ileum enterocytes. This is where B12 is absorbed. 3. In the blood, B12 is transported mainly by transcobalamin (TC1), although some is also transported by haptocorrin (TC2). 3. The B12-carrier complexes are carried around the body by the blood and distributed to tissues where they are used (e.g. bone marrow, neurons and the liver).