Blood type may define stroke risk

People with AB and women with B were a little more likely to suffer one than people with O blood — the most common type, a study found. It fits with other work tying A, B and AB to more risk of blood clots in the legs and heart attacks. Blood type O also has been tied to an increased risk of bleeding, which implies less chance of clots, the cause of most strokes.

"There's increasing evidence that blood type might influence risk of chronic disease," said chief of preventive medicine at Harvard's Brigham and Women's Hospital. The study was presented at an American Heart Association conference. The study involved 90,000 men and women that have been studied for more than 20 years.

Men and women with AB had a 26 percent increased risk of stroke compared to those with type O. Women but not men with B blood had a 15 percent greater risk compared to those with O. What's the explanation? Blood type depends on proteins on the surface of red blood cells. A pattern of immune system responses forms early in life based on them. Certain blood types may make red cells more likely to clump together and stick to the lining of blood vessels, setting the stage for a blood clot.