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How are the A, B, AB, and O blood types determined by red blood cell antigens?

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Close-up illustration of multiple human red blood cells with their characteristic disc shape, floating against a red background. — blood group The major component of human blood is the red blood cell.

The ABO blood group system classifies human blood based on the presence or absence of specific antigens on the surface of red blood cells (erythrocytes). These antigens are carbohydrate chains attached to glycolipid and glycoprotein molecules on the cell membrane.1

The determination of blood type depends on the activity of specific enzymes called glycosyltransferases, which add sugar residues to a core precursor substance:1

Type A: Individuals possess the A antigen, formed when the A gene directs the addition of N-acetyl-d-galactosamine to the H antigen.1
Type B: Individuals possess the B antigen, formed when the B gene directs the addition of d-galactose to the H antigen.1
Type AB: Individuals possess both A and B antigens on their red blood cells.2
Type O: Individuals lack both A and B antigens on their red blood cells.2

These antigens are inherited and remain present throughout an individual's life.2