Acid is released by parietal cells in the stomach's proximal two-thirds (body). Bile is produced by liver cells called cholangiocytes that line the bile ducts. Acid and bile are stored in the distal third of the stomach. Parietal cells contain granules called parietal cells secretory granules that fuse with each other to release their contents into the surrounding tissue. These granules contain the enzyme histidine decarboxylase (HDCA), which leads to the production of histamine when it is exposed to oxygen. Histamine stimulates the muscle cells of the gastric wall to contract. This results in pressure changes that cause the muscularis propria layer of the stomach to expand and contract, leading to the release of acids. An autoimmune disorder can lead to the loss of parietal cells, resulting in pernicious anemia. People with this condition cannot make vitamin B12 and therefore cannot produce normal amounts of red blood cells or white blood cells. They are at risk for anemia, cognitive impairment, and nervous system problems if they do not receive treatment. Pernicious anemia usually occurs in people who are age 60 or older. It is more common in women than men.