B12 deficiency due to loss of intrinsic factor can also be autoimmune - I forget whether the autoimmunity is against only the protein, or the cells themselves though.

The fraction of B12 converted into methyl-cobalamin, which I've read is the main or perhaps form used by the brain, may also be insufficient for optimal brain function, given all the heavy metals we accumulate in a lifetime, stress weakening the HPA axis, loss of a regular light/dark cycle affecting the SCN, pineal, pituitary, and other things.