Anemia is defined clinically as a decrease in the concentration of hemoglobin in the blood for the age and gender of an individual. While in certain anemias this decreased concentration of hemoglobin is due to a decrease in the amount of hemoglobin in each cell, most anemias are caused by a reduction in the number of erythrocytes. Causes of anemia include loss of blood (hemorrhage), insufficient production of erythrocytes, or accelerated destruction of erythrocytes in the circulation. Insufficient dietary iron or deficiencies of vitamins such as vitamin B12 or folic acid can lead to decreased production of erythrocytes. Gastric atrophy, as a result of autoimmune disease, with concomitant destruction of the parietal cells that secrete intrinsic factor, a molecule essential for absorption of vitamin B12 by cells in the ileum, is the cause of a form of anemia called pernicious anemia.
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