a classification system for the antigens of human blood; used in blood transfusion therapy; four groups are A and B and AB and O

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 Ig M 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 apes such as chimpanzees, bonobos and gorillas.