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
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