The stomach lining is supposed to produce “intrinsic factor,” which is required for B-12 absorption. Apparently, h. pylori in half of infected people causes the immune system to attack the stomach rather than just h. pylori. H. pylori evades the immune system in the vast majority of infections, but apparently the immune system code itself to target a small portion of the way h. pylori appears. Unfortunately, this portion also happens to be the part that looks just like key stomach cells, and the body destroys itself as it is attempting to kill h. pylori.
Thanks for added information on B-12 absorption. Had heard of this but did not understand the linkage. I possibly still only have shallow understanding.