Pernicious anemia is caused by a lack of vitamin B12 in the body. The main reason for the vitamin B12 deficiency is the 
loss of parietal (pa-RI-e-tal) cells in the lining of the stomach. These cells make intrinsic factor, which helps the body absorb vitamin B12 in the small intestine. In some people, the body's immune system may attack and destroy the parietal cells. Doctors don't know exactly why or how this happens, or if the immune system produces antibodies in reaction to normally aging or dying parietal cells.
As a result of this immune system attack, the stomach lining shrinks, and the parietal cells in the lining of the stomach disappear. The stomach stops producing intrinsic factor. Over time, vitamin B12 deficiency develops.
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