Anemia is a condition where red blood cells are not providing adequate oxygen to body tissues. There are many types and causes of anemia. (See the general article - anemia.) Pernicious anemia is caused by a lack of intrinsic factor, a substance needed to absorb vitamin B12 from the gastrointestinal tract. Vitamin B12, in turn, is necessary for the formation of red blood cells.

Pernicious anemia is a type of megaloblastic anemia.

Intrinsic factor is a protein the body uses to absorb vitamin B12. When gastric secretions do not have enough intrinsic factor, vitamin B12 is not adequately absorbed, resulting in pernicious anemia.

Because vitamin B12 is needed by nerve cells and blood cells for them to function properly, pernicious anemia causes a wide variety of symptoms, including fatigue, shortness of breath, tingling sensations, difficulty walking, and diarrhea.

Other causes of low levels of intrinsic factor (and thus of pernicious anemia) include atrophic gastric mucosa, autoimmunity against gastric parietal cells, and autoimmunity against intrinsic factor.

Absence of intrinsic factor itself is the most common cause of Vitamin B12 deficiency. Intrinsic factor is produced by cells within the stomach. In adults, the inability to make intrinsic factor can be the result of chronic gastritis or the result of surgery to remove the stomach. The onset of the disease is slow and may span decades.

Very rarely, infants and children are found to have been born lacking the ability to produce effective intrinsic factor. This form of congenital pernicious anemia is inherited as an autosomal recessive disorder. (You need a defective gene from both parents to get it.) However, most often, pernicious anemia and other forms of megaloblastic anemia in children results from other causes of Vitamin B12 deficiency or other vitamin deficiencies.

Although a juvenile form of the disease can occur in children, pernicious anemia usually does not appear before the age of 30. The average age at diagnosis is 60 years. In fact, one recent study revealed that nearly 2 percent of individuals over 60 years old suffer from pernicious anemia. Furthermore, slightly more women than men are affected. The disease can affect all racial groups, but the incidence is higher among people of Scandinavian or Northern European descent.

Risk factors include a family history of pernicious anemia, Scandinavian or Northern European descent, and a history of autoimmune endocrine disorders. Pernicious anemia is seen in association with some autoimmune endocrine diseases such as type 1 diabetes, hypoparathyroidism, Addison's disease, hypopituitarism, testicular dysfunction, Graves disease, chronic thyroiditis, myasthenia gravis, secondary amenorrhea and vitiligo.

In addition to pernicious anemia, other causes of Vitamin B12 Deficiency include:

• Nutrition (strict vegetarians without B12 supplementation, poor diet in infant, or poor maternal nutrition during pregnancy)
• Infection (intestinal parasites, bacterial overgrowth)
• Gastrointestinal disease (stomach removal surgery, celiac disease (sprue), Crohn's disease)
• Drugs (colchicine, neomycin, tuberculosis treatment with para amino salicylic acid)
• Metabolic disorders (methylmalonic aciduria, homocystinuria)