Blood Type-Disease Link Explained
Doctors first began to notice a connection between blood types and different diseases in the middle of the 20th Century, and the list has continued to grow. They noticed for example that people with blood type A are at a higher risk of certain types of cancer, such as pancreatic cancer and leukaemia, smallpox infections, heart disease and severe malaria. On the other hand researchers at the University of Toronto people with type O are better protected against severe malaria but are more likely to get ulcers and ruptured Achilles tendons. These links between blood types and diseases have a certain arbitrariness about them, and scientists have only begun to work out the reasons behind some of them. Their studies indicate that the protection occurred because immune cells have an easier job of recognising infected blood cells if they’re type O rather than other blood types. Take norovirus. This nasty pathogen is the bane of cruise ships, as it can rage through hundreds of passengers, causing violent vomiting and diarrhoea. It does so by invading cells lining the intestines, leaving blood cells untouched. Nevertheless, people’s blood type influences the risk that they will be infected by a particular strain of norovirus. The solution to this particular mystery was found in the fact that blood cells are not the only cells to produce blood type antigens. They are also produced by cells in blood vessel walls, the airway, skin and hair. Many people even secrete blood type antigens in their saliva. Noroviruses make us sick by grabbing onto the blood type antigens produced by cells in the gut. Yet a norovirus can only grab firmly onto a cell if its proteins fit snugly onto the cell’s blood type antigen. So it’s possible that each strain of norovirus has proteins that are adapted to attach tightly to certain blood type antigens, but not others. That would explain, researchers theorize, why our blood type can influence which norovirus strains can make us sick. Scientists say this may also be a clue as to why a variety of blood types have endured for millions of years. Our primate ancestors were locked in a never-ending cage match with countless pathogens, including viruses, bacteria and other enemies. Some of those pathogens may have adapted to exploit different kinds of blood type antigens. The pathogens that were best suited to the most common blood type would have fared best, because they had the most hosts to infect. But, gradually, they may have destroyed that advantage by killing off their hosts. Meanwhile, primates with rarer blood types would have thrived, thanks to their protection against some of their enemies.