The clinical manifestations of pernicious anemia are believed to be wholly attributable to a deficiency of vitamin B12. This deficiency is not the result of improper diet but is caused by impaired absorption of the vitamin from the gastrointestinal tract. For vitamin B12 to be absorbed in adequate amounts, a substance present in the secretion of the normal stomach is required. This substance, the intrinsic factor of Castle, has not yet been identified or obtained in pure form. The mode of action of intrinsic factor is unknown, but virtually all evidence indicates that it simply facilitates the absorption of vitamin B12. The basic defect in pernicious anemia is an abnormality of the stomach which fails to produce intrinsic factor. As a result vitamin B12 is not absorbed even though ingested in usual amounts. Similar deficiency states are encountered with certain other disorders of the alimentary canal in which there is
McINTYRE PA, SACHS MV, KREVANS JR, CONLEY CL. Pathogenesis and Treatment of Macrocytic Anemia: Information Obtained with Radioactive Vitamin B12. AMA Arch Intern Med. 1956;98(5):541–549. doi:10.1001/archinte.1956.00250290001001
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