Blood Type Defined

Blood consists of red and white blood cells, platelets, and plasma (the goop in which everything sits). Antigens and various proteins float in the plasma and on red blood cells. An antigen is any substance that causes the immune system to produce antibodies to fight intruders.

The ABO grouping system refers to genetically-determined individual differences in the presence of two antigens (A and B), which stimulate the production of different antibodies. Type-O blood has both the antibodies produced in type-A and type-B, whereas type-AB has neither.

In 2004 researchers from University College London proposed that the presence of certain bacteria and intracellular viruses may have put evolutionary pressure on certain antigen-producing genetic mutations. In populations where viruses prevailed, gene O dominated. Those with bacteria-heavy environments found themselves more likely to have A or B type.

The major blood groups were not known until the early 1900s. Before then blood transfusions sometimes were fatal, because the different blood groups are incompatible. In 1940, experiments on Rhesus monkeys revealed additional antigen factors now known as positive or negative "Rh factors." This led to the types "O positive" or "AB negative." Since then, hundreds of other less-significant antigen differences have been identified, most of which do not lead to transfusion problems.