Researchers in England dug up the bodies of people who had died from the 1665 London plague, and made the startling discovery that most of the people who died had type B blood, and that far fewer people have type B blood now than before the plague. Scientist now must explain why people with Type B blood were more likely to die from plague.

When a germ gets into your system, you make a protein called an antibody that attaches to it and kills it. Your body is never supposed to make antibodies against itself. Your immunity recognizes a germ by the sugar surfaces on cell membranes. Type B surface membrane on red cells is the same as the surface membrane of certain bacteria, including Yersinia pestis, the flea-borne bacteria that caused the plague. If you are type B, you cannot make antibodies against type B surface of cells, because you would destroy all your red blood cells. If yersinia gets into your bloodstream, your immunity does not kill it, so it causes blisters and swollen lymph nodes, and you can go into shock and die.

People in blood group A are at higher risk for heart attacks than those in other groups because their blood is far more likely to clot. They have higher blood levels of factor VIII, which helps blood clot, and clots are the terminal factor that blocks arteries to cause a heart attack. There are more than 300 known variations in blood types, and scientists are still learning about their effects.