disease in which the production of red blood cells (erythrocyte) (erythrocytes (erythrocyte)) is impaired as the result of the body's inability to absorb 12 (vitamin B12), which is necessary for red blood cells to mature properly in the bone marrow. Pernicious anemia is one of many types of anemia, a disease marked by a reduction in red blood cells or in the oxygen-carrying substance hemoglobin found in those cells. Symptoms of pernicious anemia include weakness, waxy pallor, shortness of breath, rapid heartbeat, unsteady gait, smooth tongue, gastrointestinal disturbances, and neurological problems. Pernicious anemia is in most cases associated with an inflammation of the stomach called autoimmune gastritis. An absence of hydrochloric acid in gastric secretions (achlorhydria) is also characteristic of pernicious anemia. The anemia may become severe before the disorder is diagnosed, because the vitamin deficiency develops very gradually.

In pernicious anemia vitamin B₁₂ is unavailable due to a lack of intrinsic factor, a substance responsible for intestinal absorption of the vitamin. In a healthy person intrinsic factor is produced by the parietal cells (parietal cell) of the stomach, the cells that also secrete hydrochloric acid. Intrinsic factor forms a complex with dietary vitamin B₁₂ in the stomach. This complex remains intact, preventing degradation of the vitamin by intestinal juices, until it reaches the ileum of the small intestine, where the vitamin is released and absorbed into the body. When intrinsic factor is prevented from binding with vitamin B₁₂ or when the parietal cells are unable to produce intrinsic factor, the vitamin is not absorbed and pernicious anemia results. This is believed to stem from an autoimmune reaction in which the malfunctioning immune system produces antibodies (antibody) against intrinsic factor and against the parietal cells.

Without an adequate amount of vitamin B₁₂, the body is unable to synthesize DNA properly. This in turn affects red blood cell production: the cells divide, but their nuclei remain immature. These cells, called megaloblasts, are for the most part destroyed in the bone marrow and are not released to the circulation. Some megaloblasts mature to become large red blood cells called macrocytes; they reach the circulation but function abnormally. A deficiency of white blood cells ( leukopenia) and of platelets ( thrombocytopenia) is also seen in the blood.

Pernicious anemia occurs most often in persons older than 30 years of age, although a juvenile form of the disease does occur, usually in children younger than 3 years of age. The disease shows a familial tendency and is more common in individuals of northern European descent.

Treatment involves a monthly intramuscular injection of vitamin B₁₂ that must be continued for life. Most patients improve quickly, although neurological damage is seldom fully reversible and atrophy of the parietal cells and achlorhydria persist. Before the discovery of treatment in the 1920s, the modifier pernicious, although something of a misnomer today, was appropriate, since the disease was usually fatal.