WordReference Random House Unabridged Dictionary of American English © 2017
ABO system, 
  • Physiologya classification of human blood based on the presence on the surface of red blood cells of one or both of two specific antigens, designated A and B, or their absence, designated O: every person has one of four possible inherited blood types, A, B, AB, or O, and has antibodies circulating in the blood that clump with any antigen that is not of that type, persons with type O thus being able to donate blood devoid of the antigens for transfusion to types A, B, and AB but able to receive only blood of type O.
    • 1940–45

    Collins Concise English Dictionary © HarperCollins Publishers::

    ABO system n
    1. a system for classifying human blood on the basis of the presence or absence of two antigens on the red cell membrane: there are four such blood types (A, B, AB, and O)

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