ABO system

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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