Researchers have spent years investigating whether the ABO blood group system—A, B, AB, and O—is associated with cancer risk. While your blood type is only one small biological factor among many, research has found some interesting patterns.
Among the four major ABO blood groups, blood group O is generally associated with a lower risk of several cancers compared with non-O blood groups, particularly stomach and pancreatic cancer. However, this does not mean that people with type O are protected from cancer or that people with other blood groups will necessarily develop it.
So what does the research actually show?
What Is the Connection Between Blood Type and Cancer?
Your ABO blood type is determined by inherited genes that control which antigens are present on the surface of your red blood cells.
Researchers have discovered that these blood-group characteristics may also be relevant to other biological processes, including inflammation, cell adhesion, immune responses, and interactions between cells and their surrounding environment.
Because cancer involves complex changes in cells and tissues, scientists have investigated whether differences associated with ABO blood groups could influence the likelihood of developing particular cancers.
Large observational studies have indeed found associations—but an association is not the same as proof that blood type causes cancer or prevents it.
Blood Group O Often Shows the Lowest Risk
The strongest and most consistent findings have involved blood group O.
A large systematic review and meta-analysis involving 89 studies and more than 100,000 cancer cases found that people with blood group O had a lower overall cancer risk compared with non-O groups. The analysis also found particularly notable associations with several individual cancers.
For example, compared with non-O blood groups, type O was associated with lower risks of:
- Gastric cancer
- Pancreatic cancer
- Colorectal cancer
- Breast cancer
- Some other gastrointestinal and reproductive cancers
The strength of the association varied considerably by cancer type, however.
This is an important point: there is no single “cancer-proof” blood group.
Pancreatic Cancer: One of the Strongest Associations
The relationship between ABO blood type and pancreatic cancer has attracted particular attention.
A large cohort study involving approximately 1.6 million blood donors in Sweden and Denmark found associations between ABO blood type and several cancers, with particularly consistent findings involving pancreatic and gastric cancers.
A more recent systematic review examining outcomes among people who already had pancreatic cancer also found that patients with blood type O had better overall survival than those with non-O blood types. In adjusted analyses, type O was associated with a lower risk of death compared with non-O groups.
However, this finding concerns statistical associations, not a guarantee that type O protects an individual from pancreatic cancer.
What About Stomach Cancer?
Stomach cancer is another area where the evidence has been relatively consistent.
Research has repeatedly found that blood group A is associated with a higher risk of stomach cancer compared with blood group O.
An umbrella review of systematic reviews found that blood group A was associated with increased gastric-cancer risk, while blood group O was associated with a lower risk compared with non-O groups. The researchers classified the evidence for these associations as highly suggestive.
Another large study from Taiwan followed 339,432 people and found that blood group A was associated with a higher incidence of stomach cancer compared with blood group O.
One possible explanation being investigated is the relationship between blood-group biology and factors involved in stomach disease, including interactions with Helicobacter pylori, a bacterium associated with stomach ulcers and stomach cancer. Research has found differences in H. pylori-related factors among blood groups, although the complete biological explanation remains uncertain.
Does Blood Group A Mean Higher Cancer Risk?
Not necessarily overall.
It would be misleading to say:
“People with blood group A will get cancer.”
That is not what the research shows.
Instead, some studies have found statistically higher risks for particular cancers among people with blood group A.
A large 2023 meta-analysis that included 127 publications, approximately 20 million participants, and more than 231,000 cancer patients found associations between blood group A and several cancers, including stomach, pancreatic, colorectal, liver, ovarian, cervical, bladder, breast, oral, and nasopharyngeal cancers.
But these associations are generally modest compared with major established cancer risk factors such as tobacco use, certain infections, obesity, alcohol consumption, ultraviolet radiation, and age.
What About Blood Groups B and AB?
The picture becomes more complicated when researchers compare B and AB with the other blood groups.
Some studies have found that blood group B or AB may be associated with lower risks of particular cancers, while other studies have found increased risks for certain cancer types.
For example, a prospective study of more than 18,000 Chinese men found that blood group B was associated with a lower risk of overall cancer compared with blood group A. The same study found differences in gastrointestinal, stomach, colorectal, and bladder cancer risks between blood groups.
Another large cohort study found that blood group AB was associated with a slightly lower overall cancer risk, while specific associations differed depending on the cancer being examined.
This is why it is too simplistic to rank the four blood groups from “best” to “worst” for cancer.
Why Might Blood Type Matter?
Scientists are still investigating the mechanisms.
Several possibilities have been proposed, including effects on:
- Inflammation
- Immune-system activity
- Cell adhesion
- Blood-clotting pathways
- Interactions between cells
- Tumor-cell behavior
- Susceptibility to certain infections
The ABO gene influences molecules found on cell surfaces and in other biological systems. These molecules may affect how cells interact with each other and with their surrounding environment.
However, researchers have not established one simple mechanism that explains all of the observed cancer associations.
In other words, we know there are statistical relationships, but scientists are still working out exactly why they exist.
Does Blood Group O Prevent Cancer?
No.
This is perhaps the most important message to take away from the research.
Having blood group O does not make someone immune to cancer.
A person with type O can still develop stomach cancer, pancreatic cancer, colorectal cancer, breast cancer, lung cancer, or any other cancer.
Likewise, having blood group A, B, or AB does not mean that a person will develop cancer.
Blood type is only one factor among a very large number of genetic, environmental, lifestyle, infectious, and age-related factors involved in cancer development.
Blood Type Is Not a Cancer Screening Test
Knowing your blood group can be interesting, but it should not change recommended cancer screening.
For example, someone with type O should not assume they can skip medically recommended screening because their blood type is statistically associated with lower risk.
Likewise, someone with type A should not assume that they need unnecessary cancer tests simply because some studies found increased relative risks for particular cancers.
Cancer screening should be based on established guidelines, age, personal and family history, symptoms, and other recognized risk factors.
The Difference Between Relative and Absolute Risk
Another reason these findings can be misunderstood is the difference between relative risk and absolute risk.
Suppose researchers discover that one group has a 20% higher relative risk of a particular cancer.
That does not necessarily mean that 20 out of every 100 people will develop the disease.
The actual increase depends on the underlying risk of that cancer in the population.
This distinction is extremely important when reading dramatic headlines about blood type and cancer.
What Researchers Really Know
Taken together, the evidence suggests that ABO blood type is associated with cancer risk, particularly for certain cancers.
The most consistent pattern is that:
Blood group O tends to have a lower risk of several cancers compared with non-O blood groups, while blood group A is associated with higher risks of some cancers, especially stomach and pancreatic cancer.
But these relationships are not universal.
Different studies have produced different results for some cancer types, and the size of the associations can vary between populations.
A large umbrella review also found substantial variation and uncertainty across many blood-group/health associations, highlighting why individual findings should not be interpreted as definitive predictions for a particular person.
What Actually Has a Much Bigger Impact on Cancer Risk?
Your blood type is something you cannot change.
Fortunately, many important cancer risk factors are modifiable.
Depending on the cancer, important preventive measures include:
Don’t Smoke
Tobacco is one of the most important preventable causes of cancer.
Avoiding tobacco products can substantially reduce the risk of multiple cancers.
Maintain a Healthy Weight
Excess body weight is associated with an increased risk of several cancers.
Maintaining a healthy weight through balanced nutrition and regular physical activity can contribute to overall cancer prevention.
Limit Alcohol
Alcohol consumption is associated with several cancers. Reducing alcohol intake can lower risk.
Protect Your Skin
Excessive ultraviolet exposure increases the risk of skin cancer. Sun protection and avoiding artificial tanning are important preventive measures.
Get Recommended Screenings
Screening can detect certain cancers or precancerous changes before they become more serious.
The appropriate screening schedule depends on your age, sex, personal history, family history, and country-specific recommendations.
Pay Attention to Persistent Symptoms
Unexplained weight loss, persistent bleeding, unusual lumps, prolonged changes in bowel habits, persistent difficulty swallowing, or other concerning symptoms should be discussed with a healthcare professional.
The Bottom Line
Researchers have found meaningful associations between ABO blood groups and the risk of certain cancers.
Among the major blood groups, type O is generally associated with the lowest risk for several cancers, particularly when compared with non-O blood groups. The evidence is especially notable for pancreatic and stomach cancers.
However, this does not mean that blood group O prevents cancer.
Your blood type is a biological characteristic—not a prediction of your future health.
The best way to reduce cancer risk remains focusing on established preventive measures: avoiding tobacco, maintaining a healthy lifestyle, limiting alcohol, protecting yourself from excessive UV exposure, getting appropriate vaccinations and screenings, and seeking medical advice when concerning symptoms persist.
So, if you have blood group O, you may have a statistically lower risk of some cancers—but you should still take cancer prevention and screening seriously. And if you have blood group A, B, or AB, there is no reason to panic. Blood type alone cannot determine whether you will develop cancer.