People with Type O blood have a much higher risk of developing stomach ulcers.

The link between blood type and gastric health was first noted in a landmark 1954 study published in the British Medical Journal. This research established that individuals with blood group O are significantly more susceptible to peptic ulcers. The biological reason lies in the way Helicobacter pylori bacteria interact with the human body. H. pylori bacteria use specialized proteins called adhesins to latch onto the stomach wall. One specific adhesin, known as BabA, binds directly to the H-antigen found on the surface of stomach cells. Because Type O individuals have a higher concentration of these H-antigens, the bacteria can colonize their stomachs more effectively. Research conducted at Washington University School of Medicine in the 1990s further confirmed this mechanism. Their studies showed that the bacteria's binding affinity to Type O cells is significantly stronger than to Type A or B cells. This increased 'stickiness' leads to more persistent infections and higher levels of inflammation. While about 50% of the world's population carries H. pylori, not everyone develops an ulcer. However, the genetic predisposition of Type O individuals creates a perfect environment for the bacteria to cause damage. This discovery has helped doctors better understand why certain patients are more prone to chronic gastric issues than others.