ABO blood group system

Classification of blood types

The ABO blood group system is used to denote the presence of one, both, or neither of the A and B antigens on erythrocytes (red blood cells). For human blood transfusions, it is the most important of the 48 different blood type (or group) classification systems currently recognized by the International Society of Blood Transfusions (ISBT) as of June 2025. A mismatch in this serotype (or in various others) can cause a potentially fatal adverse reaction after a transfusion, or an unwanted immune response to an organ transplant.

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