Blood type is something most people learn once and rarely think about again, unless they are donating blood, having surgery, or filling out a medical form. But researchers have long studied whether the ABO blood groups may also be linked to certain health patterns, including cancer risk.
Several large population studies over the years have found a modest association between blood group and some cancers. The pattern most often reported is that people with blood group O appear to have a slightly lower risk for several cancers when compared with people who have A, B, or AB blood types.
What Researchers Have Found
The clearest associations have been reported for stomach cancer and pancreatic cancer. In many analyses, blood group A has been linked with a somewhat higher risk than blood group O. Some estimates have placed stomach cancer risk for people with type A at nearly one-fifth higher than for those with type O.
That does not mean blood type causes cancer by itself. These studies look at patterns across large groups of people, and the results can vary depending on the population being studied. In some regions, including parts of China, researchers have seen differences in the pattern, which suggests that ancestry, genetics, environment, and local health factors all play a role.