ABO Blood Group System: Antigens, Blood Types, and Transfusion Compatibility

The ABO blood group system classifies human blood based on the presence or absence of two antigens—A and B—on the surface of red blood cells. These antigens are inherited from both parents and determine a person's blood type.

Four Main Blood Groups

• Group A: Red cells carry only A antigens.
• Group B: Red cells carry only B antigens.
• Group AB: Red cells carry both A and B antigens.
• Group O: Red cells carry neither A nor B antigens.

The Rh Factor

In addition to A and B antigens, the Rh factor (positive or negative) creates eight common blood types: A+, A−, B+, B−, AB+, AB−, O+, and O−.

Clinical Importance

Matching donor and recipient blood types is essential for safe transfusions. Incompatible transfusions can trigger a potentially fatal immune reaction.