La anemia perniciosa es un tipo de anemia megaloblástica causada por una gastritis atrófica y la subsiguiente pérdida de las células parietales del estómago responsables de segregar el factor intrínseco. Esta anemia es resultado del déficit de vitamina B12 debido a un defecto en la absorción, ya que el factor intrínseco es esencial para la absorción de vitamina B12, es uno de sus principales desencadenantes. En un 90% de los casos se debe a una reacción autoinmune. La anemia perniciosa se observa como resultado secundario de ciertas enfermedades endocrinas autoinmunes, tales como diabetes tipo 1, hipoparatiroidismo, enfermedad de Addison, hipopituitarismo, enfermedad de Graves, tiroiditis crónica, miastenia grave, y vitíligo.También por problemas digestivos o por medicamentos que alteren la absorción gástrica, como por infecciones por Helicobacter pylori, gastritis crónicas, Enfermedad de Crohn, vegetarianos estrictos que no consumen suficientes alimentos enriquecidos con vitamina B12 ni un suplemento de vitamina B12, pobreza, desnutrición, Esprue o enfermedad celíaca, una dieta pobre en el embarazo (sobre todo en el primer trimestre), en pacientes bajo tratamientos para tuberculosis, sobre todo los que toman ácido paraaminosalicilico, colchicina, neomicina. La esposa del presidente norteamericano Abraham Lincoln, tenía síntomas de anemia perniciosa y tal vez murió de ello.

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Pernicious anemia, also known as vitamin B12 deficiency anemia, is a disease in which there are not enough red blood cells due to a lack of vitamin B12. The most common initial symptom is feeling tired. Other symptoms may include shortness of breath, pale skin, chest pain, numbness in the hands and feet, poor balance, a smooth, red tongue, poor reflexes, and confusion. If treatment is not provided, some of these problems may become permanent. Although pernicious anemia technically refers to cases resulting from not enough intrinsic factor, it is often used to describe all cases of anemia due to not enough vitamin B12. Lack of intrinsic factor is most commonly due to an autoimmune attack on the cells that make it in the stomach. It can also occur following the surgical removal of part of the stomach or from an inherited disorder. Other causes of low vitamin B12 include a poor diet, celiac disease, and a tapeworm infection. When suspected, diagnosis is made by blood and, occasionally, bone marrow tests. Blood tests may show fewer but larger red blood cells, low numbers of young red blood cells, low levels of vitamin B12, and antibodies to intrinsic factor. Pernicious anemia due to lack of intrinsic factor is not preventable. Vitamin B12 deficiency due to other causes may be prevented with a balanced diet or with supplements. Pernicious anemia can be easily treated with either injections or pills of vitamin B12. If the symptoms are severe, injections are typically recommended initially. For those who have trouble swallowing pills, a nasal spray is available. Often treatment is lifelong. Pernicious anemia due to autoimmune problems occurs in about 1 per 1000 people. Among those over the age of 60 about 2% have the condition. It more commonly affects people of northern European descent. Women are more commonly affected than men. With proper treatment most people live normal lives. Due to a higher risk of stomach cancer, those with pernicious anemia should be checked regularly for this. The first clear description was by Thomas Addison in 1849. The term "pernicious" means "deadly," and was used as before the availability of treatment the disease was often fatal.

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