IF is a protein produced by cells in the stomach. After you consume vitamin B-12, it travels to your stomach where it binds with IF. The two are then absorbed in the last part of your small intestine.
In most cases of pernicious anemia, the body’s immune system attacks and destroys the cells that produce IF in the stomach. If these cells are destroyed, the body can’t make IF and it can’t absorb vitamin B-12.
Without enough vitamin B-12, the body will produce abnormally large red blood cells, which are called macrocytes. Because of their large size, these abnormal cells may not be able to leave the bone marrow, where red blood cells are made, and enter the bloodstream. This results in a decrease in oxygen-carrying red blood cells in the bloodstream. This can lead to fatigue and weakness.
Pernicious anemia is a type of large cell, or macrocytic, anemia. It’s sometimes called megaloblastic anemia because of the abnormally large size of the red blood cells produced.