ABO blood groups

The ABO blood group system is the most important blood type system (or blood group system) in human blood transfusion. The associated anti-A antibodies and anti-B antibodies are usually IgM antibodies, which are usually produced in the first years of life by sensitization to environmental substances such as food, bacteria and viruses. ABO blood types are also present in some animals, for example cows and sheep, apes such as chimpanzees, bonobos and gorillas.
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ABO blood groups A system in which human blood is classified by whether the red blood cells contain A or B antigens. [more]ABO blood groups - Community and Resources

The major blood group system. A person can be A, B, AB, or O.