The surface of our red blood cells have proteins, called antigens, which define our blood type. Type A blood contains Type A antigens and type B blood contains Type B antigens. There are 35 blood groups in the world, but the two most common groups are ABO group and Rh group. These groups contain the eight blood types that dominate over 90% of the world's population.
Odds are you are either O+ of A+, which account for 65% of all human types. However blood type can vary from one location to the next. For example type O blood is far more common in the western hemisphere. While types A and B show up more often in the eastern hemisphere.
Blood types aren't unique to humans - dogs, cats, horses, and monkeys are just a few animals that have them too.
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