Atrophic gastritis (also known as Type A or Type B Gastritis more specifically) is a process of chronic inflammation of the stomach mucosa, leading to loss of gastric glandular cells and their eventual replacement by intestinal and fibrous tissues. As a result, the stomach's secretion of essential substances such as hydrochloric acid, pepsin, and intrinsic factor is impaired, leading to digestive problems, vitamin B12 deficiency, and megaloblastic anemia. It can be caused by persistent infection with Helicobacter pylori, or can be autoimmune in origin. Those with the autoimmune version of atrophic gastritis are statistically more likely to develop gastric carcinoma, Hashimoto's thyroiditis, and achlorhydria.
Type A gastritis primarily affects the body/fundus of the stomach, and is more common with pernicious anemia.
Type B gastritis (most common overall) primarily affects the antrum, and is more common with H. pylori infection.
Read more about Atrophic Gastritis: Pathophysiology, Associated Conditions, Classification, Causes