Pernicious anemia: Low red blood cell count caused by inadequate vitamin B12. Abbreviated PA. Patients with PA do not produce intrinsic factor (IF), a substance that allows the body to absorb vitamin B12 from foods. The resulting inadequacy of vitamin B12 hampers the production of red blood cells. PA can be treated by injection of vitamin B12: oral administration will not work because people with PA cannot absorb orally administered vitamin B12. Also known as Addison's anemia.