There are many forms of anemia such as pernicious anemia. Pernicious anemia is when your body cannot absorb vitamin b12 from food you eat, as your body attacks the cells in your stomach(1). This is an auto-immune disease.
It is a complex disorder and consists of hematological, gastric and immunological alterations(2). The inability to absorb b12 vitamins stems from lack of a protein called Intrinsic factor(IF) that is made in stomach. The lack of IF occurs as a result of the activity of body immune's system that attacks and destroys cells that produce IF.There are also other factors that play a key role in b12 deficiency such as strict vegetarian diet, infections, surgery.
Pernicious anemia is also called macrocytic anemia. When there is a defficiency in b12 vitamin, the body produces abnormally large red blood cells called marocytes. Due to such large size, macrocytes will not be able to leave bone marrow, where they are made, and enter bloodstream. This decreases the amount of oxygen-carrying red blood cells in the bloodstream and can lead to fatigue and weakness.