Pernicious anemia (also known as Biermer's anaemia or Adison's anaemia or Addison-Biermer anaemia) is a form of megaloblastic anemia due to vitamin B-12 (vitamin B12) deficiency, caused by impaired absorption of vitamin B-12 due to the absence of intrinsic factor in the setting of atrophic gastritis, and more specifically of loss of gastric parietal cells. While the term "pernicious anemia" is sometimes also incorrectly used to indicate megaloblastic anemia due to any cause of vitamin B-12 deficiency, its proper usage refers to that caused by atrophic gastritis and parietal cell loss only.