B12 deficiency is not confined to one organ. The documented effects concentrate in two systems — the nervous system and blood production — with several other tissues involved.
The nervous system
NIH ODS documents neurological changes including numbness and tingling in the hands and feet, and lists dementia among the effects of deficiency.
The clinically important detail is the one NIH ODS emphasizes: these neurological symptoms "can occur without anemia, so early diagnosis and intervention is important to avoid irreversible damage." Nervous tissue is where the lasting harm happens, and where the window for intervention matters.
Bone marrow and blood
NIH ODS describes megaloblastic anemia — abnormally large, structurally abnormal, sometimes nucleated red blood cells — plus low counts of white cells, red cells and platelets. This reflects impaired cell division in the marrow.
The digestive tract
Glossitis of the tongue is documented, and the relationship runs both ways: gut conditions cause B12 deficiency as well as resulting from it. NIH ODS notes that people with stomach and small intestine disorders such as celiac disease may be unable to absorb enough B12, and that deficiency rates are higher in celiac disease.
The stomach as the upstream cause
NIH ODS describes pernicious anemia as "an irreversible autoimmune disease that affects the gastric mucosa and results in gastric atrophy," destroying the parietal cells that make intrinsic factor. It reports pernicious anemia in about 15–25% of older adults with B12 deficiency, and atrophic gastritis affecting 8–9% of adults aged 65 and older.
Reproductive and developmental effects
NIH ODS notes that in pregnant and breastfeeding women, B12 deficiency "might cause neural tube defects, developmental delays, failure to thrive, and anemia" in the infant, and lists infertility among the effects in adults.
Why this warrants a doctor, not a supplement
The irreversibility of neurological damage, and the fact that the underlying cause is often a malabsorption condition rather than diet, are both reasons this belongs with a clinician. Supplementing without identifying the cause can also mask the anemia while nerve damage continues.
Our free assessment covers the risk factors worth raising.
References
This article summarizes published guidance from the sources linked above. It is general information, not medical advice, and it cannot tell you whether you personally have a deficiency. Only a clinician ordering the right test can do that. If you have symptoms that concern you, speak to a doctor.