Autoimmune gastritis (AIG, autoimmune metaplastic atrophic gastritis)

Autoimmune gastritis affects the stomach, specifically the acid-producing parietal cells in the upper stomach lining. It is understood to result from a T-cell mediated immune response directed against the parietal cell proton pump, working alongside autoantibodies against parietal cells and intrinsic factor, a protein needed to absorb vitamin B12. Over time this immune activity destroys parietal cells, so the stomach lining thins, acid production drops, and B12 absorption falls, a process closely tied to pernicious anemia, which can develop later.

A 2024 clinical review estimated the prevalence of autoimmune atrophic gastritis at somewhere between 0.3 and 2.7 percent of the general population. It was once thought to mainly affect older women of Northern European descent and is now recognized across many populations and ethnic groups.

Common signs

Often there are few or no digestive symptoms for a long time. Some people notice indigestion, early fullness, or bloating. Because the stomach struggles to absorb iron and B12, the first clues are sometimes iron-deficiency anemia or, later, the fatigue, numbness or tingling, and balance changes that signal a developing vitamin B12 deficiency.

This is a plain-language summary for orientation, not a diagnosis. Mechanisms are described as current understanding, not settled fact. Work with your own clinician for your care.