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why does achlorhydria increases the risk for developing adenocarcinoma?
It's not the decrease in stomach acid (achlorhydria) per se that increases the risk, it's the reason behind the achlorhydria.

What's really happening is chronic inflammation for some reason that leads to atrophic gastritis. This can be the result of an autoimmune process that attacks the parietal cells, or from H Pylori infection. Either way, you get this chronic inflammation and destruction of parietal cells.

A general rule in pathology is that whenever you increase inflammation and cell turnover, you increase the risk of cancer. That holds true here. It also happens that this same inflammation and cell destruction leads to decreased acid production.
 

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why does achlorhydria increases the risk for developing adenocarcinoma?
H+ is a "gastric defense" tool to kill bacteria/viruses/etc. just like lysosymes in the saliva.

No Acid? H. Pylori (g-ve rod, motile, WHO Class 1 carcinogen) can go and sit in the submucosa, under the mucus glands, release urease (breaks down urea to yield AMMONIA << Alkaline environment (uric acid is ACIDIC, ammonia is ALKALINE) for the bacteria to swim around in) and this infection causes DYSPLASIA and increases the risk for subsequently developing GASTRIC ADENOCARCINOMA.

Dysplasia - due to chronic "irritation" is the key event. This change triggers certain latent GENE ACTIVATION whose products SHUT DOWN APOPTOSIS (Immortal cells) > GASTRIC ADENOCARCINOMA.

This bacteria CANNOT survive in acid, which explains why many, don't get H PYLORI easily.

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SECOND MECHANISM

H. Pylori causes recruitment of B-CELLS which try to contain the infection. Chronic irritation again, causes certain GENE ACTIVATION/MUTATION which IMMORTALIZES THEM - causing B-CELL LYMPHOMA (AKA MALToma) on the mucosa!

Hope this helps :)
 
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