Intrinsic factor is a protein produced by cells in the stomach lining. It is needed for the intestines to absorb vitamin B12 efficiently.
Immune system damage to the stomach lining can cause a lack (deficiency) of intrinsic factor. This can lead to vitamin B12 deficiency and low red blood cell count (anemia), which is called pernicious anemia. Intrinsic factor deficiencies can be caused by other reasons, such as surgical removal of the stomach, inherited conditions, and other diseases.