The Intrinsic Factor IgG Antibody test is a blood test that measures the level of IgG antibodies to intrinsic factor, a protein that is necessary for the absorption of vitamin B12 in the small intestine. The test is typically ordered to investigate the cause of vitamin B12 deficiency, which can lead to a wide range of symptoms such as anemia, fatigue, neurological problems, and gastrointestinal disturbances. Intrinsic factor IgG antibodies are produced by the immune system when it mistakenly identifies intrinsic factor as a foreign substance and attacks it. This autoimmune reaction can impair the body’s ability to absorb vitamin B12, leading to deficiency. The Intrinsic Factor IgG Antibody test is often used in conjunction with other tests, such as the serum vitamin B12 level and the Schilling test, to diagnose the underlying cause of vitamin B12 deficiency.