Dictionary:

ABO system

  (ā'bē-ō')

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A classification system for human blood that identifies four major blood types based on the presence or absence of two antigens, A and B, on red blood cells. The four blood types (A, B, AB, and O, in which O designates blood that lacks both antigens) are important in determining the compatibility of blood for transfusion.

A classification system for human blood that identifies four major blood types based on the presence or absence of two antigens, A and B, on red blood cells. The four blood types (A, B, AB, and O, in which O designates blood that lacks both antigens) are important in determining the compatability of blood for transfusion.

The noun has one meaning:

Meaning #1: a classification system for the antigens of human blood; used in blood transfusion therapy; four groups are A and B and AB and O
  Synonyms: ABO blood group system, ABO group