Pronunciation: /ˌeɪ biː ˈəʊ ˈsɪstəm/ · Part of speech: noun · Field: haematology / transfusion medicine

Definition. The ABO system is a system of classifying blood groups. It sorts human blood into types according to the presence or absence of certain markers (antigens) on the surface of the red blood cells, and it is the most important system used in matching blood for transfusion.

The four blood groups. Under the ABO system, blood is divided into four main groups based on which antigens the red cells carry:

• Group A — red cells carry the A antigen; the plasma contains anti-B antibodies.
• Group B — red cells carry the B antigen; the plasma contains anti-A antibodies.
• Group AB — red cells carry both A and B antigens; the plasma contains neither anti-A nor anti-B antibodies.
• Group O — red cells carry neither A nor B antigen; the plasma contains both anti-A and anti-B antibodies.

Antibodies and compatibility. A key feature of the ABO system is that people naturally have antibodies against the antigens they do not possess. If incompatible blood is given — for example, group A blood to a group B recipient — these antibodies attack the transfused red cells, causing them to clump together (agglutinate) and break down. This reaction can be severe or fatal, which is why ABO matching is essential before transfusion.

Universal donor and recipient. Because of this antibody pattern:

• Group O (specifically O negative) is often called the universal donor, since its red cells carry neither A nor B antigen and are less likely to provoke a reaction.
• Group AB is often called the universal recipient, since its plasma lacks both anti-A and anti-B antibodies and can therefore receive red cells of any ABO type. (These terms apply chiefly to red cell transfusion and are simplifications, since other blood-group systems also matter.)

The Rh (Rhesus) system. The ABO system is almost always considered together with the Rh (Rhesus) factor, another important blood-group system. A person is described as Rh-positive or Rh-negative depending on whether they carry the RhD antigen, giving familiar combined types such as A positive or O negative.

Clinical relevance. ABO grouping is fundamental to safe blood transfusion, organ transplantation, and the management of pregnancy (where blood-group incompatibility between mother and baby can sometimes cause problems). Cross-matching blood before transfusion ensures donor and recipient are compatible.

Etymology. The system is named simply for the antigens it classifies — A, B, and O — where “O” originally denoted the absence of A and B antigens. It was discovered by Karl Landsteiner in the early twentieth century, work for which he received the Nobel Prize.

⚠️ Disclaimer: This explanation is provided for general educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional regarding any clinical questions.