According to studies involving the cases of people who develop cancerous stomach ulcers, people with blood type A are more often affected by these ulcers. The reason for this is still a mystery. However, people with blood type O are more often affected by duodenal ulcers like the ones mentioned in this article.
The reason type O patients more often have duodenal ulcers is most likely related to the fact that they don't have the protective substance on cells that people with other blood types produce. This substance may be in part responsible for protecting the lining of the stomach, and thus preventing stress ulcer symptoms.